The senior politician in a department is the Secretary of State and member of the Cabinet. The Ministers of State are responsible for functions within the department. The press may refer to them both as Ministers.
Thank you. And both need to be shadowed?
Opposition shadows are often seen as shadowing individuals (or their posts). However, officially they are their party's spokesperson for a department or topic. The Conservatives are the official opposition, and Helen Whately is Shadow Secretary of State for Transport and member of the shadow cabinet. There are three other, "junior", shadows for the
DfT» - Sir Alec Sherbrooke is Shadow Minister of Transport, and is likely to take over some topics from his SSoS. Greg Smith and Dr Kieran Mullen are Shadow Parliamentary Under Secretaries - a pretty lowly form of parliamentary life, really.
The Lib Dems are not part of the official opposition, and are too few in number to name shadows on a "man-for-man marking" basis. Some do get officially listed, per department, and Baroness Randerson is the one for transport. There has to be someone in each house, and Wera Hobhouse (née von Reden) covers not only transport but energy and climate change and is Shadow Leader of the House (so a member of Ed Davey's shadow cabinet). Precise titles are a bit irrelevant with so much multihattedness going on.