BPPA associate director Howard Dawber will advise on Westminster and Whitehall developments. All three will report to SPTA director-general Malcolm Reed.
The initial six-month account - the first of its kind tackling Scottish transport issues - was awarded after a four-way pitch, against The Communication Group, Grayling Political Strategy and Politics International.
'We will work across a range of issues affecting transport in Scotland from the three policy centres. This will include rail policy and issues over Railtrack,' said Bruce.
The SPTA is the only strategic transport authority in Scotland. It runs the Glasgow Underground, invests in the rail network, subsidises buses and ferries and co-ordinates school transport operations.