Original sentence: Instead of reboring the tunnels, the extra layers were laid on the inner surfaces of all seven tunnels, narrowing the bore, and thereby ensuring that specially-constructed rolling stock was required from then on; electrification of the line was also delayed for many years..  Electrification was finally completed in 1987, using standard rolling stock, and the expedient of singling the track through the narrow tunnels
Edited sentence: Instead of reboring the tunnels, the extra layers were laid on the inner surfaces of all seven tunnels, narrowing the bore, and thereby ensuring that specially-constructed rolling stock was required from then on; electrification of the line was also delayed for many years..  Electrification was finally completed in 1987, using standard rolling stock, and the expedient of singling the track through the narrow tunnels - a simple solution that seemingly evaded railway planners for over 100 years