When the speed gets set in to a constant level.

the sentence means the following: having a load of 2,000 t and 3 power units results in huge speed fluctuations. This would consdierably reduce the effects of trying to maintain even speed patterns (keeping all trains running at the same speed). By contrast, with a load of 1,000 t this phenomenon is less pronounced, so it is this weight that has been chosen as the basis for the operating schedule.

Research has been done into how to get a maximum number of trains through on any given line. It has shown, among other things, that one of the best ways is to keep train speeds regular, in other words NOT to operate a very fast passenger train followed by a very slow freight train (on mixed traffic lines i.e. not dedicated to high-speed passenger trains only). Widely fluctuating speeds only lead to frequent stops and starts (Einbrüche) and wasted time in the process. Obviously a very heavy train will be slower than a lighter one and be more likely to create this phenomenon, preventing the smooth flow of traffic and maximum use of capacity in so doing. OK?
I was the interpreter for the working party that discussed all this

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Note added at 2004-09-22 07:53:39 (GMT)
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For David: I thought of \"slowing\" but in this context is is more than that: it is any change in speed that breaks the even pattern, for example when you get stacking at the entrance to a station because there is no platform free to receive and incoming train. The same applies on the open line where sometimes trains have to slow to maintain their distance from the train ahead. Anyone who has travelled by train must have wondered at times why it has halted in the middle of fields .... well this is why!

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Note added at 2004-09-22 07:54:02 (GMT)
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AN incoming train

is the proper term, IMO, as when a train slows down when climbing a hill. Fluctuations can occur anywhere, and are not so pronounced in their effects.

Perhaps loss to underline that speed reductions are unwanted.

"Einbruch" is a noticeable loss or reduction. In economics it could be a collapse or slump, but here I reckon it is a drop in speed or loss of speed. Perhaps it means that the heavier train will slow down considerably on slopes, even with 3 engines pulling it.

German term (edited): Geschwindigkeitseinbruch

aus erster Hand vom Eisenbahnfanatiker :)

slows down more often is my reading