Empty first class seats on selected Virgin Trains East Coast services are being auctioned through a smartphone app shortly before departure.
Prices start from Â£5 for a seat in first class carriages featuring complimentary food and drink, free wi-fi, reclining leather seats and extra legroom.
Upgrades are initially available on some weekday services from London King's Cross to Edinburgh, Leeds, Newcastle and York, with more routes expected to be added shortly.
Regular walk-up first class fares for London to Edinburgh journeys can cost almost Â£90 more than standard class tickets.
Each auction opens approximately two-and-a-half hours in advance and closes around half an hour before the train departs.
Suzanne Donnelly, commercial director at Virgin Trains East Coast, said: "We are really excited to be partnering with Seatfrog to offer customers more opportunities to experience first class.
"Passengers can get upgrades from as little as Â£5 and there are real deals to be had.
"The live bidding element means you can decide at the last minute if you fancy treating yourself."
Seatfrog chief executive Iain Griffin said people wanting to guarantee first class travel should buy a regular ticket, as auctions will not be held for all trains.
He told the Press Association: "For passengers who travel standard class and maybe want to experience first class now and again, or be a little spontaneous on the day, then Seatfrog is the perfect platform.
"We give them the chance to get a really good deal."
Mr Griffin said he is in discussions about rolling out the service with a number of other travel firms.
"I'd be really interested in working with any train service in the UK," he explained.
"I see this working perfectly for a business like Eurostar. We're already talking to rail operators in mainland Europe.
"Outside of rail, we've also got a big airline capability."
Passengers who want to use the system for a first class upgrade enter their booking reference into the app and start bidding.
Successful participants are instantly sent a new ticket direct to their smartphone or tablet.