Amtrak Siemens Charger locomotive (SC44, ALC42, ALC42E)

Help Support Amtrak Unlimited Discussion Forum:


Jul 4, 2006
Brookfield, Connecticut, USA
as I said before both Genesis and Charger have gap anticipation software, reducing traction when contact is getting lost on leading shoes.
with AC traction its a different game, as power is not interrupted as abrupt as with DC traction.


Mar 30, 2011
I don't know about the US locomotives, but in Britain, dual power locomotives were typically restricted to lower power when on third rail. For example both the Class 92 and first generation Eurostars could only reach full power when drawing AC from the catenary. The DC third rail restricted the power to something like two thirds (speaking from memory so please don't quote me on that figure). For EMUs this restriction was never a problem as they had distributed power gear and the dual system units could AFAIK always achieve the same performance on DC as on AC.

The DC-only locomotives (classes 70 and 71) had an on-board "booster" unit which was basically a flywheel to store energy. At times of peak power draw they would draw power from both third rail and from the flywheel at the same time, so limiting the peak amount of juice that had to come out of the third rail at any moment. The same flywheel also helped with gapping and even running shorter distances away from the third rail. They could also startup the flywheel by plugging into a ground supply which was useful for staring them up in the morning when they had been parked away from the third rail (most locomotive sheds were not electrified for safety reasons). The running lines were all electrified with only EMUs in mind and so in places there were/are longer gaps in the third rail that would have been too long for a locomotive. Some freight yards had DC catenary on account of the danger posed by the third rail to ground staff. Both classes had pantographs for this purpose.

On the newer classes 73 and 74 (the 74s were actually rebuilt 71s) they didn't fit a flywheel, but used a diesel engine instead. But this was quite small and although the engine could technically run any distance away from the electrified line, the effectively moderate power on diesel meant this only happened rarely. Recently some 73 have been upgraded with more powerful diesel engines and are being used on sleeper trains in Scotland, far away from any DC electrification.

I understand that in the 1980s when Class 73s were inroduced on the Gatwick Express as replacement for EMUs, they had trouble because the frequent gaps on the Brighton line led to the diesel engines being started up and shut down continuously which lead to overheating and failures and even at least one nasty fire. I think they fixed that by tinkering with the software and also trained the drivers to better anticipate the gaps.