I have tried to build transformers underground on the Bridgewater-Brunel server, but to no avail.
This thread relating to Standard may be relevant, although it seems to conflict slightly with what is now in the code:
https://forum.simutrans.com/index.php/topic,14543.msg143947.html#msg143947 (https://forum.simutrans.com/index.php/topic,14543.msg143947.html#msg143947)
I have the same issue, but with the underground tube signalbox.