Go-Ahead Group PLC (LSE:GOG) revealed it is dealing with a cyber security incident following unauthorised activity on its network on Monday.
The transport group immediately had external forensic specialists on the case and took precautionary measures with IT infrastructure as soon as the issue was brought to light, it said in a statement.
It will continue to investigate the nature and extent of the security threat, with incident response plans in place.
Go-Ahead insisted there is no impact on UK or international rail services, which have remained operating as usual.
The company said it has notified regulators, including the Information Commissioner's Office in the UK, as a safety measure.