Originally occurred on my server with 112-3 and pak 128, confirmed with nightly r6585 and pak 64.
Building powerline tunnel entrances normally and with CTRL-click works fine, and then in underground mode you can click and drag from the powerline as you would with a normal tunnel, but if you press control and drag the powerline tunnel, Simutrans will crash when you release the mouse.
Confirmed.
Edit: fixed in 6587:
r6587 | kierongreen | 2013-07-12 23:43:16 +0100 (Fri, 12 Jul 2013) | 1 line
FIX: crash when constructing powerline tunnels and not keeping faster ways (i.e. pressing control)
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