Informed Sources

With electric traction reaching Swindon it’s BOTTOMS for the time being

In the inal Intercity Express Programme Train Technical Speciication there is quite a lot on Automatic Selective Door Operation (ASDO), but no mention of the need for the bi-mode trains to change between electric and diesel traction, and vice versa, automatically at linespeed. In those prelapsarian times it was assumed that bi-modes would run to the extent of electriication, change over to diesel in the station and continue on their way.

That was before the implosion of electriication on the Great Western, which saw politicians reaching for ways to cut costs. In particular, it was argued that the money went into raising structures to provide the necessary clearance to earth for the Overhead Line Electriication (OHLE) equipment. So why not leave these structures in place and use bi-mode trains, which could drop the pantograph and run through tunnels and other height restrictions under diesel power?


Thus, a new abbreviation emerged – APCO, or Automatic Power Change Over. And, as reported in this column (November 2017), the irst opportunity to test this concept emerged when Network Rail was refused permission to replace the listed Steventon High Street Bridge west of Didcot.

Here, the problem is not the clearance under the bridge itself, where Special Reduced Clearance has been used, but the fact that, approaching the bridge, the adjacent Stocks Lane level crossing requires the contact wire to be at maximum height, before reducing to the minimum height in about a quarter of a mile. As the contact wire falls, it is pushing the pantograph head down and for every action there is an equal and opposite reaction, which is not good for either the catenary or the pan.

Controversial structure: an IET passes under the B4019 road bridge at Steventon.
Brian Perryman


The point behind this question is that DfT is the System Integrator for the Intercity Express Programme trains.

Andy McDonald (Shadow Transport Secretary): To ask the Secretary of State for Transport, pursuant to the Answer of 19 October 2018 to Question 178321, what assessment he has made of the role of his Department in overseeing the introduction of the Automatic Power Change Over (APCO) system integration (a) across the Intercity Express Programme and (b) at the Signalling Scheme Plan at Steventon Bridge.

Jo Johnson (Transport Minister):

DfT is working together with the Rail Industry to enable facility for Automatic Power Change Over (APCO) on Intercity Express Trains on the East Coast and Great Western routes. This includes appropriate power change over arrangements at Steventon Bridge. I understand that Network Rail have launched a formal appeal to the Planning Inspectorate against the local authority decision to refuse planning permission for replacing Steventon Bridge. Written Parliamentary Answer 1 November 2018

Maximum acceptable contact wire slope, or gradient, at such height changes is determined by the speed of the train. The rule of thumb is that the steepest gradient is ive times the speed in miles per hour. For 125mph the steepest gradient is thus 1:625. But the proximity of bridge and crossing at Steventon imposes a gradient of 1:270.


As a result, and pending Network Rail’s appeal against the Council’s decision, a Permanent Speed Restriction of 60mph has been imposed on electric traction.

This would cost time, given that Class 800s should be running at 125mph at this point. While in theory you could drop the pan and coast through, there would still be a loss of speed. Instead, bi-modes will switch to diesel power to run under the bridge.

This could seem an ideal application for APCO. Just put down some standard Eurobalises, as already used for the European Train Control System (ETCS) through the Thameslink central core and for Tilt Authorisation and Speed Brian Perryman Supervision (TASS) on the West Coast main line. Then use the Packet 44 message facility to take passing Intercity Express Trains (IETs) through the power change-over sequence automatically.

Well, APCO has been tested on both the Great Western and East Coast main lines and it works.

But there is a big diference between switching from electric to diesel at some point on a main line and coping with a short diesel intermission on an 125mph electriied, bi-directional line.

As a result, rather than APCO it will be a case of BOTTOMS at Steventon. That is Back On To The Old Manual System.

This has meant providing signage for the driver. The box shows the signage required. Note that a chain is 22 yards so that there are 80 to a mile.


In the down direction the change from electric to diesel mode is made manually at Moreton Cutting (51M 53ch) east of Didcot, in part to reduce the driver workload. Having passed under Steventon Bridge the reversion to electric traction occurs in the half-mile long ‘safe pantograph raising area’ starting at 57M 40ch.

Note that the driver is warned of the changeover at 57M 18ch with changeover initiated at 57M 51ch. The driver then has until 57M 65ch to press the button.

At 125mph it takes 0.36 seconds to cover a chain, so the ‘window’ between the ‘change to 25kV’ sign and ‘do not raise pantograph’ is ive seconds. Hmm.


According to the signalling Scheme Plan, approaching from the west there are APCO balises on both tracks at 59M 20ch, at 57M 65ch and also either side of the warning boards at 57M 18ch. Each balise has a unique identity.

In efect APCO requires just three balises on a line to carry out the demanding manual process, with its risk of error. At which point we should note the proven ability of those on the front end to perform at levels of reliability exceeding human factors predictions.

According to informed sources, APCO is due to be implemented at Newbury in December.

Meanwhile the balises on the up main approaching Steventon, which initiate the change to diesel traction, have been tested successfully. There have also been successful tests at Temple Hirst on the East Coast main line.

Going into the detail of manual changeover and APCO has brought home to me that ‘discontinuous electriication’ is not a sensible or practical long-term solution. While the Class 800s are smart enough to do it for themselves, another sign at Steventon will read ‘EMU 60’.

Hopefully Network Rail’s challenge will see the bridge demolished and raised, but on future electriication schemes procuring bi-modes – more expensive to buy, operate and maintain – to avoid clearance issues should not be an option. Of course, future electriication schemes depend on bringing down electriication costs and any day now the tectonic plates could shift.


Bridge is at approximately 56M 32ch Safe pantograph raising area is between 57M 40ch and 58M


57M 18ch Down Direction: Warning of traction change-over

57M 51ch Down Direction: ‘Traction change-over to 25kV’

57M 65ch Down Direction: (Modiied) ‘Do not raise pantograph’

59M 20ch Up Direction: Warning of traction change-over

58M 62ch Up Direction: ‘Train class speciic instruction’ for Class 80x trains bearing legend ‘Diesel’

57M 58ch Up Direction: traction change-over to diesel with legend ‘Class 80x’


57M 18ch Down Direction: Warning of traction change-over

57M 51ch Down Direction: ‘Traction change-over to 25kV’

57M 65ch Down Direction: ‘Do not raise pantograph’

59M 20ch Up Direction: Warning of traction change-over

58M 62ch Up Direction: ‘Train class speciic instruction’ for Class 80x trains bearing legend ‘Diesel’

57M 58ch Up Direction: traction change-over to diesel with legend ‘Class 80x’