1979 British Rail Advanced Passenger Train – So Near, Yet So Far

(first posted 7/8/2017)      Don’t let people tell you British Rail was a slow, stodgy and conservative organisation, unable to adapt and change with the times. Well, not all the time. In 1976, it gave us the 125mph High Speed Train, still in frontline service today, and still unmatched – a true transport great. And just five years later, we could travel on the first practical tilting train in the world, and it was capable of 155mph. But learning that development of the Advanced Passenger Train was started before the HST suggests it isn’t a simple story.

By the mid-1960s, British Railways (BR) had come a long way from nationalisation in 1948. Steam was almost gone, replaced by a very mixed bag of diesel power and with just one modern electrified route – the West Coast Main Line from London Euston to Birmingham, Manchester, Liverpool and Glasgow. The network had shrunk by a third, as surplus and duplicating routes were ruthlessly eliminated. But passenger traffic was dropping, as motorways and domestic air travel began to steal business. Something was needed to get speeds above the 100 mph of the WCML electrics and the mighty Deltics never mind the paltry 90mph achieved elsewhere, if intercity passengers were to be won back.

Unlike Japan and France who built new lines just for high speed passenger trains for the famous Shinkansen (‘Bullet’) and TGV (Train a Grand Vitesse) trains, Britain lacked the funds and political appetite to develop a new high speed network, so faster trains would have to be accommodated on the existing infrastructure.

But how to get speeds up without building new lines?

First, the physics. The biggest constraint on the average speed of a modern train isn’t power, it’s curvature. A train can’t take curves like cars or bikes, because of the centrifugal force that tries to push it outwards, with disconcerting effects for passengers well before the train gets thrown off the rails. This can be tackled in two ways – reduce speed, or raise the rail on the outer side of the curve above the inner one, a technique called ‘super-elevation’, as seen here.

But super elevation is a compromise. It must be enough to allow faster trains to avoid disruptive slowing, but not too much to give passengers on slower trains the sensation of being tipped inwards to the curve. So, in the mid-1960s, even with super elevation and electric acceleration, expresses on the West Coast Main Line faced frequent speed restrictions, not just on the 200 notoriously sinuous miles between Preston and Glasgow but also on some of the sharper curves further south. These constrained London – Glasgow times to a minimum of 5 hours, at an average of 80mph, which wouldn’t be enough to compete with car and plane into the 1980s.

The seemingly obvious answer to that dilemma is to tilt the train into the curve, to keep the effect of the train’s mass in line with the track. This was the solution put forward by a specially recruited research team in BR – a proposal for a train that would tilt through corners, thereby increasing cornering speeds by 50% and thus able to reach and sustain higher cruising speeds. No conservative bogged down by tradition thinking here

Tilting has downsides of course. Because of the restrictions of the standard BR loading gauge (the envelope of length, height and width that a train must fit within), the body of a train that tilts (blue in the drawing above) would need to be narrower above the waist than conventional trains (yellow) – without the distinctive inward slope, APT would collide with bridges, tunnels, platforms and other trains when tilting. And then there was the worry about nausea induced by the tilting motion – a big concern for many people before they travel on a tilting train, and for nobody afterwards, as it doesn’t happen if you remain aware of your surroundings.

BR aimed high; speed through curves was to rise by 50%, while maximum service speed would rise from 100mph to 150mph, on existing track with existing signals and no extra track maintenance, whilst maintaining current levels of passenger comfort. Quite a tall order. Clever calculations came up with the proposition that track, train and passengers could safely and comfortably withstand a tilt of 9 degrees from the vertical, which would be enough on most curves to get the desired speed increase. So, in 1964, BR, with government funding, began the serious development of a tilting train capable of 155mph.

BR had assembled a special team of engineers for the task of designing and developing the APT. Many had aeronautical backgrounds, which allegedly influenced the extensive use of lightweight alloys in the structure. Gas turbines were chosen for the power cars – their only use on Britain’s rails, bar a brief experiment in the early 1950s. There was a purpose-built development laboratory and engineering centre at Derby, one of Britain’s great train building cities since the earliest days of the railways. And even a dedicated 13 mile test track, on the trackbed of a closed secondary line, conveniently close to Derby and with a good mixture of curves and straights that was ideal for testing a tilting train – this was a test vehicle built to experiment with tilt

The development programme was where it began to go wrong. The original plan had been for a prototype to be ready in two years, but this was hopelessly optimistic. With funding issues, and delays in selecting and proving the gas turbines, it was 1969 before the design was settled.

Fortunately for BR (and everyone else), other engineers in BR had foreseen this slow progress, and in 1970 they persuaded the Board to authorise a second project, which became the HST. Initially presented as an interim solution pending squadron deployment of the APT, the HST (seen here in prototype form, alongside the APT-E at Swindon) was running in under two years, in service within six and is still running at 125 mph 40 years later – the fastest diesel train in the world. Truly a remarkable piece of engineeerring, even if past its stylish best now.

When it finally emerged in 1972, the APT-E (Advanced Passenger Train – Experimental) looked like no train before or since – perhaps a small jetliner on wheels is the closest description.

It was built of lightweight steel, and the two power cars and the two trailers between them were carried on articulated bogies, where one bogie supports the trailing end of one vehicle and the leading end of the next, again to reduce weight.

Power came from five Leyland gas turbines, each of 300hp. This power unit could trace its lineage back through those used by Rover in experimental car installations in the early 1960s to Sir Frank Whittle’s wartime experiments. It was chosen as it provided plenty of power without the weight penalty then typical of diesel engines, and allowed the APT-E to operate away from electrified lines – it was always assumed the production train would be electric. Overall, the four car train was styled to suit its purpose – a striking pointed nose, a high single central windscreen, and a silver-grey finish offset by a BR blue stripe that ran over the roof at both ends. Overall, it was a remarkable vehicle.

Tilt was achieved through accelerometers on each bogie detecting lateral movement of the vehicle as it entered a curve, and initiating the movement of two hydraulic jacks (one on each side of each bogie) to deliver the required degree of tilt. The target was to generate tilt at 5 degrees per second. This shot, with one tilt mechanism jammed, shows what that looks like.

APT-E was not designed for passenger service, but was strictly for experimentation to prove the concept and research the problems of high speed tilt and high speed braking. The trailer cars were equipped as mobile laboratories to monitor and test systems to aerospace standards – although contemporary film shows slide rules at every work station!

And APT-E proved the concept; after early arguments with the train drivers’ union about there being only one seat in the cab, and reliability issues with the gas turbines, the performance began to improve. In August 1975, it set a UK rail speed record of 152.3mph near Swindon, and averaged over 101mph between London St Pancras and Derby – today, over 40 year later, the fastest trains average around 85mph.

APT-E was retired from its testing career in 1976, as development of the prototype APT-P began. This was to be an electrically powered train, of up to 14 coaches, powered by two centrally located power cars. Stylistically it was a big advance from APT-E, partly because of the more attractive nose treatment (and seats for two drivers) and the new dark and light grey with red striping livery, which soon appeared on the HSTs as well. And just as the HST was officially the InterCity125, APT became InterCityAPT.

Unlike the APT-E, APT-P was designed for passenger service, and the coaches were fully equipped for seating and catering use. The upholstery (in first class top, and second class) may look a little dated now, but the layout still looks better than on many modern trains, with larger windows and better seat spacing. Partly, that reflects better crash protection through a stronger structure in 21st century trains, and partly the need to cram in as many seats as possible nowadays. But it certainly looks better than a Pendolino inside and out to me (but a Pendolino is structurally very strong indeed).

A big weakness with the APT-P was the need to locate the two power cars adjacent to each other in the centre of the train, with one pantograph serving both, which effectively made it two separate trains as there was no passage through the power cars. Contemporary rules prevented the cables for the 25kv electricity being fed through the passenger vehicles, and it was believed the overhead power line would be so disturbed by the pantograph on the leading power car that the trailing one would be out of alignment, and damage would ensue if the power cars had been located at the two ends, as on the HST and the APT-E. There were also concerns about the dynamics of 8,000hp pushing a train if the power cars were both placed at one end, so the mid train position was the unhappy compromise result.

The first power car was completed in June 1977; the first trailer cars a year later, but a complete train did not run until May 1979, such was the complexity and lack of resources afflicting the project. There was some good news in December 1979, with a new UK speed record of 162.2 mph – not broken until 2002.

And APT-P had some cutting edge features beyond tilt. Hydrokinetic brakes, which are basically water turbines, were used to ensure the train could slow from 155mph within existing signal distances, while an early transponder system called Control-APT gave the driver in cab display of the maximum permitted speed in the centre of the driver’s panel, surrounded by a blue mounting

But by now, the winds were turning against the APT. A new government, led by the famously anti-rail Margaret Thatcher, was demanding a return on its investment in BR, while the decision to move to electric power meant the APT was confined to just one route, the WCML, as there were no funds for more electrification. The perceived failure of the APT stood in stark contrast to the runaway success of the HST, and BR was forced to take a huge risk to try to save the project – the prototype had to go into passenger service.

So, in December 1981, at 6am on a cold murky Glasgow morning, one of the three APT-P sets (from an original plan for eight) set out for London Euston, full of journalists and BR engineers. London was reached in 4 hours 15 minutes at a maximum of 125 mph without incident except for overhung journalists suffering from the tilt motion once the sun came up.

But the smooth running didn’t last. December is a cruel month for testing heavy engineering in public, and APT-P suffered a spate of problems (frozen hydrokinetic brakes, door problems, you name it).

But BR really tried. They made this film, presented by the BBC’s Peter Purvis, to fightback at the naysayers and the critics. It was an uphill battle, as the train was obviously not yet ready for regular service. Public service ended after less than three weeks, and the APT-P was taken away for a thorough review and modification.

It returned in1984, with no publicity, and slowly earned a reputation for reliability. But it was not used at high speed, and just mixed in with the WCML 100 mph electrics. Clearly, APT was not the train of the future anymore. But it did record the first sub-4 hour London – Glasgow journey – 3 hours 52 minutes for 400 miles of railway built for Victorian steam engines was a real achievement, and stands as the fastest rail trip between the two cities to this day.

It was finally quietly taken off the mainline in 1986; two sets were scrapped and the third sits at a private heritage centre just outside Crewe station; if you’re lucky, you’ll spot it from your Pendolino. Plans for an APT-S (Squadron) production version were quietly dropped.

Today, APT-E rests at the National Railway Museum’s Co Durham outstation, Locomotion, alongside another great express prototype, Deltic.

You might think that was the end., but the technology lived on. Much of it was used in the 4,830hp class 91 electrics built for the East Coast Mainline in 1988, which copied the APT’s unusual body (rather than bogie) mounted electric motors, which significantly reduces the unsprung weight and thus track impact. Designed to reach 140mph, or 225km/h, they were known as InterCity225 – the APT name had lost its credibility; but, despite their 140mph capability, they only reach 125mph, as the signalling improvements needed failed to materialise. The accompanying mark IV coaches were designed to accept a retrofitted tilt mechanism, and thus copied the APT’s distinctive sloping profile. Now nearly 30 years later, these trains are still the ECML’s frontline, but they don’t tilt and never will.

Ultimately, BR sold the rights to the APT tilt system to FIAT, who then developed a series of tilting trains that now run across Europe, using the Pendolino name. And, in 2001, Virgin Trains introduced a fleet of tilting trains on the WCML, built in Birmingham by Alstom with FIAT tilting technology. Yes, Richard Branson’s pride and joy is based on APT’s pioneering engineering of a quarter centrury earlier – not that he’ll tell you.So maybe BR wasn’t an old fashioned, unadventurous and uninspired organisation. Perhaps it was 20 years ahead of everyone else, but we just didn’t appreciate it.