The MP for Hertsmere met with the Minister of State for Transport to discuss a secret meeting about plans for a rail freight terminal in Radlett.
James Clappison joined Anne Main, MP for St Albans, at the meeting with Theresa Villiers, last week. The pair wanted to ask why she had met with Simon Hoare, consultant for rail freight developer Helioslough, behind closed doors to discuss the proposals.
The two MPs also questioned the minister over documents suggesting that the Department for Communities and Local Government has been lobbied behind the scenes by the Department for Transport.
Mrs Main said: "What we want is maximum transparency in the decision making process. James and I are working as a team to get to the bottom of this.
"No one wants a rail freight interchange in Radlett and we are fighting anything that might add weight to the wrong side of this decision.
"The more we probe, the more we uncover. James and I will continue to ask questions to ensure that this decision is made fairly and transparently. The developer must not be able to get the advantage or the upper hand."
The proposed terminal would be on the former site of the Radlett Aerodrome, between Park Street and London Colney.