Fewer trains have been running due to a shortage of train crew.
Buses were running on some lines including Nuneaton and Atherstone, Hednesford and Rugeley Trent Valley and Crewe and Liverpool South Parkway.
West Midlands Trains said half-term leave was a factor but also said there were a number of reasons why there may be a crew shortage.
Displaced train crew due to disruption such as flooding could also be a factor, they said.
Buses were also running between Wolverton and Milton Keynes Central.
Common reasons for train crew shortage include annual leave allocation at popular holiday times, staff training and short-term sickness, the firm said.
The company, which operates West Midlands Railway and London Northwestern Railway, has not said whether the situation will continue beyond Monday.
West Midlands Trains said where gaps in the rotas were identified, it looked to cover these with staff volunteering to work overtime or on their rest days.
A spokesperson said it apologised to its passengers whose journeys had been disrupted by cancellations on Monday and encouraged anyone delayed by 15 minutes or more "to claim compensation via our Delay Repay scheme".
Advice over alternative services was put on the National Rail Enquiries website.
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