It is a steam build. When I click revert to original, the signal asset reverts to its packaged state. If I then "open for edit" the asset and check it for errors, a whole load of errors then pop up (take for example <kuid2:60850:90141:4> signal theatre single), suggesting that the asset itself if faulty. In any case, if I have a junction with <kuid2:60850:24158:13> Sig 3AT BR Post Elec Dbl LHS just before it, <kuid2:60850:23953:3> Sig T Theatre 3 on one route, and <kuid2:60850:23957:3> Sig T Theatre 7 on the other route, then the theatre box correct appears but it doesn't light up, suggesting there is a problem with the theatre box asset or the scripting.