so hopefully what is a mystery to me will be elementary to you.
I don't know about xD
Had another look at this issue this morning; there seems to be a definite link between whether the tender is visible in the
put in front menu, and whether the tender can be used with multiple locomotives, eg. in January 1956:
The BR standard tender (49t version) can be used with a 7MT
Britannia or an 8P, and is visible.
The BR 9F tender can only be used with a 9F, and is not visible.
There was at least one exception, IIRC the GWR
Duke tender, but there was nothing obviously different in the dat file.
It will be worth seeing if the newer constraint code solves the issue. Does anyone more familiar with the constraints code understand why the above might occur?