Oooh, I shoud resist but I can't
OK, 60mph is 1 mile per minute or 1600meters per min or about 27meters per second.
A 3 car DMU. Each carriage is about 35 tons. Total weight is 105 tons or 105,000kh.
The kinetic energy of the train is E= 1/2 m v2.
So the total kinetic energy of the train at 60mph is 0.5 * 105,000 * 27 *27
This is 38,272,500 Joules.
Looking it up, the energy content of diesel is about 38MJ / litre.
So in pure energy terms, it would appear you would only need a 1 liter of diesel.
But, and it's a big one...
No engine is 100% efficient. A good diesel engine can optimistically turn 30% of the chemical energy into usable work. Allow for a well used train engine and you're probably looking at about 20% actual efficiency.
So, the extra fuel to slow down and then speed back up will be about 5 litres.
Of course, this might be complete rubbish but it seems plausible!
Cheers,
Jason