The waste, which includes soft toys, plastic bags and photographs, was discovered by the charity that operates Telford Steam Railway.
The site - owned by Network Rail - is near the A4169 near Horsehay, the Shropshire Star reports.
Network Rail said it was supporting an investigation by West Mercia Police to identify the culprits.
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Network Rail said it had been monitoring the site to find out who was responsible for the fly-tipping.
Simon Masters, media relations manager, said: "Fly-tipping causes a great amount of misery to those who live nearby, as well as costing the taxpayer tens of thousands of pounds in clean-up costs."
Paul Hughes, from Telford Steam Railway, a group which leases a signal box on the site, said he first discovered the dump before Christmas.
"We were doing a routine inspection when we discovered there was some commercial fly-tipping which progressively increased to what has become a huge mountain of waste," he said.
"We are all used to seeing somebody drop off an old sofa or bedspread but this is on a completely different scale."
Telford and Wrekin Council said it was working with the police and the Environment Agency.
Some 150 tonnes of rubbish has been dumped near a popular steam railway attraction in Shropshire.