They attacked a UK Government agency that owns the bridges across Britain for the “desperately disjointed” move when ministers were trying to encourage more “active travel”.
Bridges over former railways which have been earmarked by Highways England (HE) for closure include three on a planned path between Neilston and Uplawmoor as part of East Renfrewshire Council’s strategic cycle corridor.
Infilling a bridge at Pugeston in Angus could prevent the expansion of the Caledonian Railway Brechin heritage railway.
Others include in Aberdeenshire, East Ayrshire, East Lothian and Fife.
The HRE Group, an alliance of walking, cycling and heritage campaigners, said: "With billions being invested to encourage more of us onto bikes, it’s a sign of desperately disjointed government that HE – acting on the [UK] Department for Transport’s behalf – is seeking to demolish or infill bridges that span or carry old railway lines that have the potential to be converted into greenways.”
Member Graeme Bickerdike said: “Thousands of 19th-century bridges carry traffic over our disused and operational railway network.
"The associated risks can be managed effectively without putting them beyond use at great expense.
"Most of us recognise we need to build a better future – that’s why railway re-openings are being investigated and a huge investment made in new active travel routes.
"Existing infrastructure should not be lost blindly to HE’s stifling culture of risk aversion when it could prove valuable in years to come.”
Uplawmoor Development Trust, which is involved in the East Renfrewshire path, said it was confident that infilling the bridges on the route could be blocked.
Chair David Riddet said: “We have been aware of the proposal to infill the bridges and have registered our objection to Highways England.
“We believe that as HE would require planning permission from East Renfrewshire Council, it is highly likely that permission will be refused.
"We understand that these bridges are structurally sound.”
Caledonian Railway Brechin chairman Jon Gill said: “The short-sighted approach to infilling of bridges with limited or no consultation, and in particular this listed structure, would severely impact any future possible return to direct rail links between Brechin and Montrose.
"This could have a negative impact on local tourism, employment and attempts to reverse the impact of climate change through more efficient transport links.”
A Highways England spokesperson said: “To maintain the safety of communities living near to Historic Railway Estate structures, and the drivers who use the roads that cross them, we are planning to infill up to 23 bridges structures in Scotland over the next five years.
“All of the structures in Scotland earmarked for infilling are over 100 years old and were never designed to carry modern vehicles.
"We’ve spoken to all the local authorities to ask how they intend to manage the public safety concerns and about future plans for the structures.
"Where they have no use for the structures, infilling is the safest and most appropriate option.
“East Renfrewshire Council has indicated there are plans for the former Lanarkshire and Ayrshire railway, so there will be further discussions with the local authority about plans for three bridges.”