Train operator Govia Thameslink Railway (GTR) has apologised to passengers whose journeys were disrupted after the introduction of huge changes to services.
Passengers complained of services being cancelled when a new timetable was introduced on Sunday - one said services back to the Huntingdon and St Neots area from Hitchin had been replaced with coaches, another said her daughter had to stay with friends in Peterborough overnight and others said there had been a shortage of drivers.
There were also problems today (Monday) when commuters faced using the new trains for the first time.
Anne Hall who travels to London from St Neots and who has been critical of GTR's service, said she did not get to work until 10am, having expected to arrive at St Pancras at 07.45. Her train eventually pulled in to Kings Cross at 9.02.
The shake-up of services is one of the biggest seen on the rail network and passengers had been warned it may take time to bed in.
A GTR spokesman said: "We are implementing over the coming weeks the biggest timetable change in a generation to boost capacity and improve reliability, introducing 400 extra daily trains and space for 50,000 extra passengers in the peak. Greenwich Thameslink now gives passengers from this north Kent route improved connections across central London and, from December, Crossrail's Elizabeth Line, with two trains an hour to Luton from Rainham.
"Due to the scale and complexity of the task, these changes will be made incrementally. This involves redeploying drivers and trains and changing operating practices to achieve a large increase in the number of services, carriages and station stops."
The spokesman said: "Despite some cancellations, passengers will benefit from an overall increase in capacity with immediate effect.
"We apologise to customers for any inconvenience caused."
Mrs Hall said: "I did not get to work until 10.00. The 06.48 to St Pancras which I was going to catch was cancelled due to "operational difficulties". The next train the 06.59 to Kings Cross was 22 mins late due to "operational difficulties".
"The first train to arrive at St Neots was due to go to St Pancras which I jumped on. We got to Finsbury Park and it stayed there for about 8 minutes. Then it announced that it wasn't going to St Pancras but would be going to Kings Cross."
She said: "We got just outside Kings Cross and then they said there was a signalling problem. We then went backwards and stayed in a tunnel for a while. Eventually pulled forward and went to Kings Cross and arrived there at 9.02. I should have been in St Pancras at 07.45.
"Commuters are very angry. I would like to call on the CEO of Gt Northern/Govia Thameslink to resign."
Cllr Darren Tysoe said his 7.20 train was 10 minutes late but that he was prepared to give the new service time."
A passenger told the Hunts Post: "The new timetables are shocking. The train service has gone from bad to terrible overnight." But another passenger said: "Always been a fantastic experience travelling to London."