A reward of £50,000 is being offered in a bid to trace the arsonists who crippled Chiltern Railways and Virgin CrossCountry services between Oxford, Banbury and Birmingham.
A signal relay cabinet was set alight at Small Heath, in Birmingham, on September 25, putting signals out of action as far as Leamington Spa.
Trains have been running to an emergency timetable, which will apply until repairs are completed some time this week.
It was the eighth arson attack on railway signal equipment in the West Midlands in the past two years.
British Transport Police are investigating.
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