East Coast train operator LNER is to introduce mandatory seat reservations on all train services from Monday.
The measure, to help social distancing on trains, will mean passengers with flexible tickets will have to pick a service to travel on, in advance.
The company operates services between London and Leeds, and Edinburgh, York, Newcastle and London,
Separately, Avanti West Coast is urging customers to reserve tickets, but has not made it compulsory.
However, it says passengers may not be allowed to get on a train if they do not have a reservation.
Avanti, which runs services linking London, Glasgow, Manchester and Birmingham, is also encouraging people to wear face coverings when they travel.
Chiltern Railways is also advising its passengers to book tickets in advance where possible and to wear masks.
LNER confirmed its shift to mandatory reservations in a tweet.
This contained "tips" for passengers, travelling on its trains, including:
- Wear a facemask if you cannot keep your distance
- Ignore the seat number on your ticket and choose your own seat
- Keep 2 metres apart where possible
- One person to a row unless travelling as a household
- In standard class leave two empty rows and one in first class
- Avoid facing other passengers
The company concluded by asking: "Can you travel another way? Help us keep the trains clear for those who really need them."
To support social distancing, our services will be reservation only from Monday. We’ll have full details on our website over the weekend and here’s some tips for those customers who do need to travel with us. pic.twitter.com/Byddg2jw9R— London North Eastern Railway (@LNER) May 15, 2020
LNER said passengers would have to make a seat reservation if they have a flexible ticket that does not have a reservation, or if they are a season ticket holder.
Customers will not be charged for making a reservation.
Existing seat reservations are still valid, but will no longer be linked to a specific seat number. LNER said it was asking passengers to choose their own seat on board the train.
A spokesperson said: "The safety of our staff and customers is always our top priority. To support social distancing for customers who do need to travel with us at this time, all LNER services will be reservation only from Monday 18 May 2020.
"We will have full details updated on our website over the weekend. LNER services continue to operate for those who have no alternative travel options."
Avanti's new measures also take effect on Monday and the company said it may not allow carriages to be more than a quarter full.
"We're appealing to our valued customers to help us and other passengers by only travelling with a reservation," said Avanti West Coast's managing director Phil Whittingham.
"If everyone does this, we'll be able to keep social distancing in place on board, both for our customers and our people.
"If customers do turn up without a reservation, we'll do our best to help but we can't guarantee they'll be able to take the train they want."
The train operator is asking passengers to book in advance on the Avanti mobile app where possible, and to avoid using facilities at the station or handling cash.
People should also check before they travel, in case the time of their train has changed.
Other measures being introduced by Avanti include face masks for staff, while waiting rooms and lounges will be shut.
There will also be enhanced cleaning procedures on board trains and at stations, focusing especially on cleaning door buttons, grab handles, tables and all touch points, as well as equipment such as phones, chip and pin machines, self-service ticket machines and point of sale systems.
Shops on board Voyager services, which travel between London and destinations such as Blackpool, Shrewsbury, Birmingham, Edinburgh and North Wales, will be closed and no food and drink will be available.
The shops on Pendolino services will still be open, but re-usable coffee cups will not be accepted.
Increased train services
Avanti said a new timetable was being brought in from Monday, in line with updated travel advice from the government that will see train services increase to about 70% of the normal timetable.
During the coronavirus pandemic only half of normal rail services have been running.
Chiltern Railways also revealed a new timetable, which comes into force on 18 May, and has advised its passengers to book tickets in advance where possible and to wear masks.
The company - whose trains from London Marylebone travel on routes to Aylesbury, Oxford, Stratford-upon-Avon and Kidderminster - also told travellers to avoid rush hour and allow more time for their journey.
Meanwhile, bus operator National Express says it has begun selling coach tickets for a restart to services on 1 July, subject to government advice.
The easing of travel restrictions is likely to be done gradually - the government has suggested that working hours might be staggered to limit passenger numbers.
People in England who are allowed to return to work have been asked not to use public transport if possible.
If maintained, the two-metre social distancing measure would cut capacity on trains by up to 90%.
A recent Transport Focus survey suggested more than 60% of UK passengers would not feel comfortable using public transport unless social distancing was in place.
It found 51% would not be happy unless passengers were required to wear masks.