Lorries were banned from using a bridge with "bits falling off" after Network Rail threatened to stop running trains on the lines below, it has emerged.
Authorities in Weston-super-Mare slapped a weight limit on the Winterstoke Road crossing after it was found to have "significant defects".
Councillor Elfan Ap Rees said traffic vibrations were causing debris to fall on to the tracks below.
Vehicles over 7.5 tonnes will be diverted along the A370.
The bridge, owned by the Ministry of Defence, was built during World War Two using emergency powers to provide access to a nearby aeroplane factory .
Councillor Elfan Ap Rees said the bridge, which crosses the main Bristol to Taunton railway line, had become "too dangerous".
"The problem is bits falling off it, both off the structure and also the iron work and obviously with high speed trains running underneath that's too big a risk," he said.
"We've been discussing with the MoD for some time how they're going to finance the repairs to the bridge and they've finally had to admit they're going to need to do something because Network Rail is threatening to not run trains under the bridge."
An automatic number plate recognition system will be installed on the bridge to enforce the weight restriction.
"If we reduce the weight that will help the bridge to last longer and reduce the risk of pieces falling off on to the railway line," Mr Ap Rees said.
"But sooner or later the whole structure is going to have to be replaced," he said.
Network Rail has been contacted for comment.