By BBC Scotland's political correspondent Glenn Campbell
The Tories want Transport Minister Nicol Stephen to explain his links with the firm he has approved as preferred bidder for the ScotRail franchise.
It has emerged that the minister used to work as a consultant for Aberdeen-based FirstGroup before becoming an MSP in the area.
Mr Stephen also addressed a FirstGroup event at the Scottish Liberal Democrats' conference in Dunfermline.
His speech took place during the franchise bidding process.
Before accepting invitations like this, the ministerial code of conduct says ministers must "satisfy themselves that doing so does not place them under any real or perceived obligation nor risks the commercial position of the executive".
Transport Minister Nicol Stephen approved the bid
The Scottish Conservative's transport spokesman, David Mundell MSP, is calling for these issues to be "cleared up to ensure there's no shadow over the process".
An executive spokeswoman confirmed Mr Stephen was a consultant to FirstGroup in the mid-1990s.
In a statement, she said: "It would be improper to suggest that this may have had any bearing on the decision to appoint FirstGroup as the preferred bidder.
"The process was carried out on an anonymous basis."
FirstGroup's chief executive, Moir Lockhead, added: "The process is so clear and so independent that past relationships, even current relationships mean nothing".
A spokeswoman for National Express, which currently operates ScotRail services, said the company was "very disappointed" to be losing the franchise but had no concerns over the bidding process.
The Strategic Rail Authority was responsible for recommending a preferred bidder.
Their choice was approved by the transport minister without him being told FirstGroup was the company behind the bid.