British Transport Police officers in York are following "all lines of enquiry" after train staff reported suspicious behaviour on Monday night (August 30).
Writing on Twitter in the early hours of Tuesday (August 31), BTP North Yorkshire said train staff had reported "behaviour that was out of the ordinary", and said officers would investigate fully to "ensure the safety of everybody on the railway".
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The force shared a message from BTP Lancashire sent on Monday, reminding the public to be vigilant on the railways.
It said: "As restrictions lift, we encourage everyone to remain vigilant.
"If you see something that doesn’t look right, trust your instincts and ACT. Report it to police, security or railstaff. In an emergency call 999."
BTP North Yorkshire added: "We can't reiterate this enough."
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