Metro Trains suspects a group of disgruntled train drivers is out to sabotage the spring racing carnival, the busiest time of the year for Melbourne's rail operator, by pressuring staff to refuse overtime and call in sick.

The company is investigating claims some of its staff are waging a campaign of "bullying and harassment" against drivers who work overtime, including targeting them with "inappropriate graffiti on notice boards".

The Spring carnival is the busiest time of the year for Metro Trains. Credit:Penny Stephens

The company has written to drivers to warn them it will pursue staff involved in an "illegal" industrial campaign to reduce its pool of available drivers, which could force it to cancel trains and leave passengers stranded.

Metro has been hit by an unusually high service cancellation rate in recent days.