If I want to electrify a tunnel track, it is difficult to electrify the bit connecting the tunnel entry to the rest of the tunnel: This works only if either
(a) I click once before the tunnel, then change to underground mode and then click once in the tunnel, or
(b) if I electrify everything except the tunnel itself first, then I click on the tunnel entry and then on the tile next to it.
(pak64 at 100.0)
Intended this way and cannot be changed.