First of all, contact Weaver about the 031; they are usually helpful - I've called them
over the years with questions. If you are still running curves that tight, you would really
want to stay away from any loco that big; awkward operation and awkward appearance.
Weaver 3-rail products stick to the same "standards" that L/M/K do/did. As completely
compatible as any of them - though Weaver rolling stock plastic couplers are pretty poor, so far as their operation goes. I treat them like dummy couplers.
But: the U-boats have die-cast couplers.