The 7,000-mile Trans-Asia railway will follow one of the old Silk Roads through Istanbul, Tehran, Lahore and Delhi.

It is already being described by train buffs as “the world’s greatest railway journey” and will be longer than the Trans-Siberian railway, which spans 5,772 miles.

Under a United Nations-sponsored scheme, Pakistan and Iran will link up their lines in the coming months to join the sub-continent’s track to that of Europe for the first time.

The UN said the link would open up new trade routes within Asia and give the former Soviet republics of central Asia rail access to Iran’s strategic sea port at Bandar Abbas on the Gulf.

The route was extended when the Calcutta to Dhaka line reopened earlier this month, more than 40 years after it was blocked during the Indo-Pakistan war of 1965.

Last week, senior Indian officials met their Iranian counterparts in Tehran to discuss progress. India has already earmarked £90m to extend its vast rail network towards its border with Burma. From there just 218 miles of missing track stands in the way of an overland rail journey from London to Singapore.

An intrepid traveller will soon be able to leave London for Brussels, Cologne, Vienna, Bucharest, Istanbul, Tehran, Quetta, Lahore, Amritsar, Delhi and Calcutta before reaching the end of the line in Dhaka.

The prospect has caused excitement among Britain’s rail enthusiasts. Mark Smith, whose website Seat61.com promotes rail adventures around the world, was planning his first London to Dhaka itinerary.

His trip incorporates the Eurostar to Brussels, breakfast in Vienna and onward trains to Istanbul, where travellers must take the ferry across the Bosporus linking Europe with Asia. The ferry will eventually be replaced by an underground tunnel, but for now passengers will be able to enjoy views of the Aya Sofya and Topkapi Palace.

Smith’s journey continues with a Turkish express train to Lake Van, close to the Iraqi and Iranian borders, where passengers switch to another ferry to get to the Tehran-bound express, which is described as surprisingly modern.

Iranian engineers have extended their network through Kerman to the Pakistan border, where travellers will switch to a Pakistani train before continuing their journey to Quetta.

China, a big supporter of the project, is spending billions on extending rail lines to its Burmese border.

Trans-Asia railway sources said the only barrier to eventually connecting London to Yunnan province and Singapore was Burma’s military regime, whose poor human rights record means that no foreign funding is available to rebuild its railways.

Smith, who always books seat 61, said the journey offered a return to romantic overland adventure, despite some security concerns on the Iran-Pakistan border.

“If you have the time, a taste for adventure and can arrange the necessary tickets and visas, this promises to be a truly epic overland journey,” he said.