Commuters in Melbourne's west could get the short end of the stick under Labor's proposed $50 billion suburban rail loop, research suggests.

The massive underground suburban rail network linking every major rail line in Melbourne and the new airport rail link has been billed by the Andrews government as the "biggest public transport project in Australian history".

But while it could serve more than 195,000 commuters in Melbourne's south-east, it may benefit only about 51,000 people in the west, leading planning and economics firm SGS says.

The 90 kilometres of track would connect 10 train lines from Cheltenham in the east to Werribee in the west and is expected to take until 2050 to complete.