Speed restrictions are to be imposed across wide parts of the rail network amid warnings of a record UK heatwave from midday.
With temperatures forecast to soar into the high 30s, steel rails could top 50ºC in the sun – leaving the risk of buckling under pressure.
To protect passenger safety, speed limits are being halved on the East Coast Main Line south of Peterborough.
The speed limit of the East Coast Main Line is 125mph in normal circumstances, but will be temporarily limited to 60mph at the southern end of the line.
This is set to lead to train cancellations, longer journey times and busier services on the network. The speed limit on the Midland Main Line will also be halved south of Leicester.
Rob McIntosh, Managing Director of Network Rail Eastern, apologised for the "disruption this will cause" but said it was "essential to maintain the safety of the travelling public".
"Passenger safety will always be our number one priority. And with forecasts of unprecedented temperatures, we have to take action to keep people safe.
“Reducing speed limits will mean fewer services and busier trains on our main routes into and out of London.
“So I’d like to ask all passengers to think about whether their journey is really necessary and, if it is, to make sure they plan well ahead.
Train operating companies are working on revised schedules, but there will be reduced services, with longer journey times and the likelihood of overcrowding.
Anyone who has to travel should consult operators for the latest travel information, allow extra time for their journey and carry water with them.
It is hoped services will return to normal on Friday, with temperatures forecast to drop. There will be a blanket speed restriction of 60mph on the East Coast Main Line between Peterborough and King’s Cross from midday today.
The same speed restriction will also apply from midday on the Midland Main Line between Leicester and London St Pancras.
This will have a knock-on impact on services in Yorkshire and the North East. There will be significantly fewer services, longer journey times and those trains which are running are expected to be extremely busy.
Network Rail is urging passengers to consider whether their journey is necessary and to consider alternatives.