Hi Martin
the only 1/72 scale german locos in polystyrene that I'm aware of are by Hobbyboss, who make the BR52 Kriegslokomtive (link below) which is said to be a nice kit
http://henk.fox3000.com/hobbyboss.htmArmageddon (Mach 2) make a 1/72 scale armoured locomotive, the Panzerlok BR 57 as part of their BP-44 armoured train, but you have to buy the train with it (half or whole train) . But this kit is not cheap, and of the standard of a poor resin kit. Takes a lot of work to get this one together, poor moulding, & poor fit of parts, lots of sanding and filler needed.
http://henk.fox3000.com/Armageddon.htmalso in 1/72 scale:
UMmt make a couple of different russian WW2 armoured locomotives and train wagons.
UM make some self propelled german rail cars
ACE make a couple of russian armoured cars converted to rail.
All the above manufacturers are fairly good , though not up to Dragon or Revell's standard, so you will need to a little sanding and puttying up on some joins
The german rail gauge was (and still is) Standard Gauge, which is 4 feet 8.5 inches from the top of the inside rail edge, to the top of the opposite inside rail edge.
for metric countries (like Germany) thats 1435.0 mm.
In 1/72 scale thats 19.93 mm between the inside top edges of those rails
in 1/87 scale thats 16.49 mm which is HO rail track. This track is too narrow for a 1/72 scale model.
Some people have narrowed the axles on 1/72 train kits so they fit HO scale track.
Hobbyboss make 1/72 scale track, though it is a tiny bit overscale at about 21mm, it scales to closer to Broad Gauge.
I've found 1/72 kits of trains & vehicles made for rails come with a piece of track, so unless you intend to build a larger base or diorama, you have a piece of track in the kit to sit the model on, or mount on a small base.
Fred (Graywolfgang) re mixed scales, couldn't agree with you more, with the mess of scales out there.
good luck
cheers
Neil