 The signalling in the long and difficult tunnel section was conventional, using the Tyers Lock and Block system, but the signals themselves had no semaphore arms; instead they had a rising spectacle plate that exhibited the usual night time oil lamp colours to drivers.This subjected the Caledonian and its successor railway to criticism, as the smoky tunnel conditions, coupled with commonplace bunker-first running, led to difficulty for drivers.The intensive train service and the occasional failure of the lock and block system seems to have encouraged signalmen to adopt irregular practices to deal with the failures.