The Heritage Railway Association (HRA) says English coal supplies will run out in early 2021, with Welsh supplies lasting until 2022.
Chairman of the West Somerset Railway Jonathan Jones-Pratt said:"The whole industry is in jeopardy over this.
"We've got so much coal here but the problem is that we can't extract it."
Steam locomotives rely on bituminous lump coal to burn, which is relatively smokeless and comparatively clean.
The opencast mine at Dewley Hill, near Newcastle, would have produced this coal but it was rejected for environmental reasons.
The HRA said the decision "dashed the hopes of Britain's heritage railways, who need affordable coal to continue operating".
The only remaining mine producing lump coal in the UK is Ffos-Y-Fran near Merthyr Tydfil, which is due to close in 2022.
It is estimated the heritage railway industry used 26,000 tonnes of coal per year pre-Covid, accounting for 0.02% of the UK's carbon emissions.
Once stocks run out the industry will be reliant on foreign imports as demand from the steam industry would not be enough to sustain a domestic coal mine, according to the HRA.
Ian Crowder, from the Gloucestershire and Warwickshire Steam Railway, said: "We've burned Russian and Polish coal in the past, but it was filthy stuff and difficult to run the engine without producing a lot of smoke.
"Bringing it from overseas will be more expensive in the future and will generate a huge carbon footprint.
"We pay £200 per tonne of coal now. In normal circumstances our engines use three to four tonnes a day, even with footplate crews being economical."
Mr Jones-Pratt said the "public perception" of coal was the reason new mines were refused.
"The industry keeps being hit by these planning application refusals, but steam railways are so popular and we're always overbooked, it's just a political thing," he said.
Tom Bright, from Steam Railway Magazine, said the issue was another "nail in the coffin" for the industry on top of Covid-19.
"I'm an environmentalist, but they have to burn coal and it has to come from somewhere.
"The problem is that it's a visible burner and it looks worse than it is, but if an engine is fired properly the exhaust is very clean.
"We face a future where the Hogwarts Express, The Flying Scotsman and other famous engines will be reliant on foreign coal.
"Overseas sellers could charge what they want, it genuinely puts these engines at risk."
Some attractions, like the Severn Valley Railway, have already switched to overseas supplies.
"We'd been using British coal the past ten years but our supply ran out in mid-December," said Duncan Ballard, the railway's contracts manager.
"We are currently sourcing coal from abroad and are working with other railways to secure a sustainable and reasonably-priced source.
"One of the unexpected benefits of this is that its CO2 emissions are significantly lower than those of our previous coal."
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