I'm 98% sure that that was actually illegal and against every H&S▸ rule in the book...! That carriage should have been locked out of use...! Maybe worthy of an e-mail/letter to FGW▸ ??
Try telling this to the North York Moors Railway!
OK, they may not fall under Network Rail jurisdiction, and they may only have one tunnel on the route which isn't your average Box Tunnel lenghth tunnel, but I was up there earlier in the month with my parents and we went on the
NYMR▸ , and during the in the tunnel bit of the journey, despite there being lights in the carriages, they were not turned on on either the outward or return journey.