I'd've though the current would actually be quite high due to the length of cable between installations. You need a relatively high amperage to counter the voltage drop over distance. I believe the voltage rating for signalling cables is 650/1100v.
Errrrrrrrrrrrr no quite right
Signal power supply is 650V although there are some systems the use 400V there are supply points that step this down to the local power used by the signalling equipment typically this is 110V there are other low voltages.
The higher voltage of 650V is used to over come the volt drop due to the resistance of the feed cable, the current in the cable is a product of the load it is supplying and is driven by the EMF (Voltage)
Any cable carrying current produces heat, this becomes a problem if the cables are incorrectly sized or the protective device (fuse) is to big for the cable.
Could the damage caused by this fire been due to litter catching light or a deliberate act of vandalism, signalling Locs catching fire has happened in third rail areas due to traction bonding problems causing traction current to flow through signalling equipment but this is a non electrified area.