I've seen a few places where a crossover in NESA (National Electronic Sectional Appendix) is not electrified.
Are electric trains still allowed to use them? Do they have to lower their pantograph?
Electric trains are generally not allowed to use them, as the gap between opposing OLE is enough for the pan to spring up and risk taking the wire out of the 2nd track. Similar issues with 3rd rail trains being gapped usually mean they're not allowed to use them. I suspect that in an emergency and with a sufficiently long train some form of emergency working could be made where each pan is dropped and raised in succession but I've never heard of it happening.
Why wouldn't they be electrified?
These crossovers generally only lead to non-electrified track, or are in places where electric trains would not reverse unless emergency working was needed. The additional cable run for the crossover is still a maintenance issue, even if the cost is on the face of it marginal, and it also saves on providing shunt movements in the interlocking. It's very possible these crossovers are put in later, and if nothing electric is planned to take the crossover, why spend a vast amount more?
One example is Newhaven Town Yard, where 25 crossover could allow access for trains to terminate in the Up platform during perturbed/amended working. However this would require additional interlocking for the level crossing and additional signalling to permit the reversible movement that would be used a handful of times a year. All the trains into and out of the yard need to be diesel as they run over non-electrified track to reach their end destinations (Acton/Cliffe Brett Marine etc), so electrifying the crossover would have been a waste of money, even at the marginal costs. The option of electrifying it was being explored as a way to get more freight into and out of Newhaven Marine, but that is very much a later development and so wouldn't have been within the scope of the original installation of the yard.
I agree with GRALISTAIR. For the marginal costs it is incredibly short sighted. Some of the ECML crossovers north of Alnmouth are like this and that seems (amongst other things) to have ended any hope of using EMUs on local trains as they cannot be turned back.
Really, which crossovers between the Mains are not electrified? I think some of the sidings around Berwick aren't electrified, but I thought all loops and crossovers were, given it's bi-di (or at least SIMBIDS) up there.