When you say "size", which one do you mean: power rating, total mass, or length/width/height?

_{*Lunatic*, n.
One whose delusions are out of fashion.}

In general, Re the reason you mention for the US: in Europe, until recently, the principle was one locomotive per train. The only exception from that being mountain passes, where railways employed pusher locomotives (without coupling them); or, on the main Alpine passes, two pulling locomotives (on the Gottthard, usually an SBB Re 6/6 II and an SBB Re 4/4 II or III for a total of ten axles, called by railwaymen "a Re 10/10"). But none of these, nor the current practice of double (on normal lines) or up to quadruple heading (on Alpine passes) assumed redundance for locomotives breaking down.

Here, breakdowns were supposed to be prevented by good maintainance -- then again, trains run shorter distances, and when there is a breakdown, a replacement loco is not far (well, that is, unless bad management reduced spare locos kept in sheds justfor this purpose).

_{*Lunatic*, n.
One whose delusions are out of fashion.}

Well, I guess we are all shade tree mechanics here at ET - of one sort or another ;-)

You can't be me, I'm taken