Rail union leaders have written to Prime Minister Boris Johnson with "severe concerns" over plans to increase train services.
Transport Secretary Grant Shapps told the BBC on Sunday that more buses and trains would run as part of a staggered approach to easing the lockdown.
The letter says it is "completely unacceptable" to put passengers and rail staff at risk.
The government says workers should still stay at home where possible.
"Our advice is clear that the best way to protect our NHS and save lives is to stay home if possible," a Department for Transport (DfT) spokesperson said.
"Our rail system has been carrying key workers and freight around the country since the current restrictions were put in place, however we must ensure the network is ready to respond to a change in demand when these are lifted."
Union bosses say there is currently no plan to be able to increase services while also maintaining social distancing.
"We therefore call on the government and train operators to work with us in establishing where there is a real demand to increase services and, where that demand exists, how it can be delivered safely," says the letter, signed by the general secretaries of Aslef, the RMT and the TSSA - Mick Whelan, Mick Cash and Manuel Cortes.
Last month, one rail boss told the BBC that social-distancing of any kind would be "extraordinarily difficult" to manage and police. Another said it could reduce the capacity of an individual train by between 70% and 90%.
At the moment about half of normal train services in the UK are running so that essential journeys are possible.
Stay at home
The DfT said in response it understood that talks would be needed to work out how to increase services.
"Reinstating services is a complex and time-consuming task, which is why we are in talks with the rail industry and unions on this issue," it said.
"In the meantime it is imperative people continue to follow the government advice and stay home and only use public transport if you have to."
Earlier, Mr Shapps told the Andrew Marr Show that the government was looking at a range of options for people to travel to work, including encouraging what he described as a "massive expansion" in interest in "active travel" such as cycling or walking.
"There are a series of different things that we can do including staggering work times, working with businesses and organisations to do that," he said.
He also said he was working with train companies and unions on maintaining social distancing rules on platforms and at bus stops.
Eurostar passengers will be required to cover their faces from Monday or risk being refused travel.
The rail company said the rule for travellers to wear face coverings was in line with guidelines from the French and Belgian governments.
Any type of face covering is allowed "as long as it effectively covers your nose and mouth", a statement said.