Rail journeys across Bedfordshire, Buckinghamshire and Hertfordshire have been affected by the cold weather on Thursday.
East Midlands Trains and First Capital Connect (FCC) said they would be running an altered service on some routes.
The Thameslink service between Bedford and London is running a reduced route due to "heavy snowfall", FCC said.
It comes after temperatures plunged to -4C (25F) overnight.
The Thameslink service from London to Brighton and Sutton has been suspended.
East Midlands Trains said only three trains an hour would be running to and from London St Pancras.
Fell into pond
A statement on its website said all services were subject to delays and late alterations.
Forecasters have predicted a mostly dry day, with lengthy spells of sunshine. Just three schools are expected to close due to a lack of heating.
East Midlands Trains said passengers are advised to check for up-to-date travel information and leave additional time for their journeys.
Luton Airport was open on Wednesday but said departures and arrivals were being delayed by problems in other regions and countries.
The East of England Ambulance Service urged people to be aware of the dangers of frozen waters after a 12-year-old boy fell into a pond on a common in Chorleywood on Wednesday afternoon.
A spokesman said the boy was accompanied by a friend who called for help. The boy was pulled free by his mother.