London's subterranean Mail Rail service should be kept open to reduce traffic above ground, according to the London Assembly.
Mail Rail is the only service of its kind in England
The last three stations on the 75-year-old service, which runs underground from east to west London, are due to close at the end of May.
In its heyday, it carried four million letters a day and stopped at nine stations between Paddington and Whitechapel.
But changes to the distribution of mail and a massive drop in postage in the city mean it is too expensive to run, says Royal Mail.
A London Assembly report out on Tuesday says more action is needed to return the line to full service and avoid "leaving a unique part of London's heritage to gather dust".
Assembly members accept the financial pressure Royal Mail faces, but say it has not done enough to find ways to make the Mail Rail profitable again.
Their suggestion that the railway could be used to transport wine across London is the sort of imagination we can do without
Andrew Pelling said: "Taxpayers, who fund the Royal Mail, will want to see good use made of this asset.
"Even if the rail system is transformed into a victualler's delight by making it transport wine and spirits it will be doing something to keep traffic off central London streets."
But a Royal Mail spokesman said the service costs more than four times as much as the alternative above ground.
He said it would not be left to gather dust as a specialist team would keep it in good working order.
"It's hugely ironic that the GLA members recognise Royal Mail is in a perilous financial position and needs to save money, then criticises us when we take action," he said.
"Their suggestion that the railway could be used to transport wine across London is the sort of imagination we can do without - but any sensible ideas will be welcomed."
He added that the service is funded by Royal Mail revenues from stamps and other products.