Originally Posted by: mike c 
European version of the 10VA Start Set transformer.
I don't see any pictures from TS, so you know things I don't, for example loc 3074 and transformer number and type 37540.
If you have loc and transformer number, it's easier to retrieve information.
There is no coincidence between an analog transformer (220, 230 volt) and the pulse on an analog loc.
(OK, just have to adjust the spring tense a bit, perhaps, intuitively).
There is a rather complex technical explanation for it (about changed electric current fluctuation on the European net),
and there is simple practical information (I know people using these old blue transformers with analog locs, without problem).
But don't use that transformer on a digital loc, the decoder will be burned and never works again,
whatever the voltage the transformer was meant for.
Analog blue and some analog white transformers are incompatible with decoder usage.
So if there might be a problem, this loc must have an electronic reversing unit (FRU in German) from either Märklin, Uhlenbrock, Hag or what so ever. And that unit then should be restricted to exactly 25 volt. I don't know any model of those. Those restrictions came during the decoder introduction.
On the contrary, I have 2 HAG reversing units (built for Buco), and they have a pulse range from 18-48 volt, caused by the relay type used to switch direction. As soon a pulse is higher than 18 volt, the (bistable) relay switches once. Capacitors prevent a next switch, based on the same pulse.
Anyway, all analog Märklin locs in the past were leaving the factory in a box with a leaflet.
(Second hand bought trains seldom have the leaflet included.)
In that leaflet was written (in 4 languages) what to do with and buy for that specific loc (reverse spring tension, oiling principles, asb change, a.s.o.).
Of this loc number (3074) I scanned 2 pictures out of that leaflet (and put them in an Album), and attached them here
(just to see how it works):
Fig 1:
Text (in Dutch):
