The government is in talks with rail bosses to put emergency measures in place to deal with falling passenger numbers after the coronavirus outbreak.
Some train operators were already losing money but fewer fares will put even more pressure on their finances.
A senior industry source said fairly drastic measures might be required for train companies to survive.
At an industry meeting last week, passenger numbers were said to have fallen by up to 18% on certain lines.
However, another industry source acknowledged that the fall in passengers could be significantly higher.
They told the BBC that the number of passengers travelling through major UK train stations at peak times had dropped considerably in recent days.
Very up-to-date figures for the whole UK network are not known as many tickets are still not purchased digitally, so it takes some time for the data to filter through.
Under franchise agreements, train companies have a range of contractual obligations, which govern how many trains they run and restrict how much they can charge for tickets.
They are also required to make payments to the government to run services on parts of the rail network.
The number of trains, the price of tickets and the amount companies pay government are all calculated based on assumptions about passenger numbers.
But, with fewer people catching the train – as some companies ask staff to work from home over fears about the spread of the coronavirus – the ability of rail companies to meet some of those obligations is now in doubt.
The BBC understands that train operators are in talks with government to renegotiate the terms of some of those contracts.
Train companies want the government to give them more wriggle room so they can keep operating services for essential travel for people working in the emergency services, even though broader passenger numbers have fallen.
Options being discussed are likely to include a reduction in the number of train services and flexibility over the payments that train companies make to government.
A spokeswoman for the Department for Transport said: “We recognise how difficult the current situation is for the transport sector and, across government, we are engaging with the sector’s leadership to support workers, businesses and passengers.”