Out on the Seymour line in central Victoria the trains are still run by a signalling system that hasn't changed since the 1870s.

It's as old as Edison's electric light bulb and, like that invention, it still works, although actual working examples are hard to find these days.

Called double line block signalling, this arcane system of bells, levers and wooden boxes has been used to safely run trains between Melbourne and Seymour since 1876.

It's the last of its kind in Australia, a steam-era communication system V/Line persists with on its fifth busiest rail line, years after it was phased out elsewhere.

But its days are numbered.

Although V/Line could not say when it will be replaced, the planned expansion of Melbourne's urban boundary will trigger the end for the primitive technology as towns such as Donnybrook and Wallan are swallowed by suburbia.

There are just a handful of staff at V/Line who know how to run the old system, which has its own language of bell calls, similar to morse code.

V/Line's Kilmore East station master, Colin Mans, is one of them.

Aged 59, he has worked for the Victorian railways since he was 16, long enough to have operated double line block signalling when it was still used on some suburban lines in Melbourne.

He doesn't want to see it phased out on the Seymour line, and argues it's more reliable than modern signalling.

"Even if something does break, it is not going to affect the travelling public," Mr Mans says.

"That's why this line has better on-time figures than all the other lines."

This is true.

The Seymour line has better punctuality and reliability figures over the past 12 months than V/Line's other, busier commuter lines to Geelong, Ballarat, Bendigo and Traralgon.

It is also the only intercity line that was left out of the Regional Fast Rail upgrade of the early 2000s, when tracks and sleepers were replaced, modern signals were installed and line speeds were increased to 160 km/h.

Seymour line trains still can't get anywhere near those speeds.

Bart Matthews, V/Line's senior maintenance manager, signals and communications, said double line block signalling is "a fascinating piece of engineering that continues to provide a safe and reliable service more than a century after it was built".

"With a range of improvements planned for the Seymour line we know it won't be around forever - but for the moment it's still fit for purpose and doing the job," Mr Matthews said.

Paul Westcott, the Public Transport Users Association's regional spokesman, said the simplicity of the old signalling technology made it more robust than modern systems, but also slower and an impediment to running more trains.

"It also tells a story of underinvestment over many decades," Mr Westcott said.

"The signalling might be in the 19th century configuration and the track and other infrastructure is very much still in the 20th century as well."

The state government's regional network development plan proposes running more frequent services on the Seymour line, which has just 20 trains a day, increasing train speeds and replacing old signalling.

The proposal is unfunded.

Late last month the government announced it would release 100,000 new housing lots on Melbourne's expanded urban fringe by the end of next year. Tens of thousands of those lots will depend on the Seymour line for public transport access to Melbourne.