New ministers always go a little mad in the early days of the job. The new railways boss, Jo Johnson, will make a speech proposing to “phase out” all British diesel engines by 2040. He calls it an “aspiration”.

But diesel accounts for a third of all Britain’s trains. The only alternative form of traction is electricity, and Johnson’s ministerial predecessor postponed or abandoned Network Rail’s electrification plan – including the Welsh and Midlands mainlines. British railway traction is in the dark ages. What does Johnson have in mind, horses?

There are murmurs that trains might be hauled by hydrogen or batteries (which would need to be the size of a train). As with cars, that is pie in the sky. But is Johnson about to cause a sensation, and revive the age of steam?

Leisure and so-called heritage railways have continued to expand, and their appeal is simple. They run steam trains

Steam is the secret gold mine of the railway. Passenger numbers on conventional trains have slackened after the great boom of the past decade – hence the financial woes of the east coast mainline. Rail fares have been soaring and commuter services have been congesting. But leisure and so-called heritage railways have continued to expand, and their appeal is simple. They run steam trains.

At the time of the Beeching cuts in the 1960s, there were barely a dozen private steam operators. Today there are roughly 120 – plus another 50 community partnerships using mostly diesel, but sometimes mixed with steam. There are more private railways in Britain than at any point in the 20th century. They are booming.

Steam engines of course burn coal, and coal is dirty. There is no way it can be “cleaned” on board a train. But nor is generating electricity entirely carbon-free. The total amount of carbon at issue is minimal, and almost all its pollution is outside towns. It could be matched by emissions certificates.

Replacing diesel with popular steam would not suit every service. But some 70% of non-commuter rail travel is leisure and tourism. On existing private lines, some operators reckon they can charge twice as much for a steam train as for a diesel. Kings of the steam sector, such as the North York Moors, the West Somerset and the Ffestiniog are now serious transport undertakings. Derelict engines have gone from being valueless to costing thousands.

The viability of these lines is clearly aided by their appeal to volunteer labour, of which there seems an inexhaustible supply, not least among ex-railway staff. They have a devoted passenger following among those who, in the modern age, are exhilarated by the sound, smell and pace of steam. So come back George Stephenson. Johnson is on to something. Phase out diesel – and bring back steam.

• Simon Jenkins is a Guardian columnist