The RMT said it was balloting its 523 London Midland conductor members over a decision to place security contractors on trains within the network without agreement.
RMT general secretary Mick Cash said: "London Midland’s attempts to place external security contractors on board trains with an already safety critically trained guard without agreement with RMT is totally unacceptable.”
He said over a two-year period, union negotiators offered a range of alternative proposals which were "dismissed out of hand every time or regretfully ignored" and that the RMT had been left with no option but to ballot for industrial action.
The ballot opened yesterday and will conclude on 2 February.
A London Midland spokesman said: “We are very disappointed that the RMT is threatening to disrupt our passengers’ journeys. We have made it clear to the RMT from the very beginning that this is about the safety and security of our colleagues and customers.
“We introduced additional security contractors based on feedback from front-line staff and passengers. The security contractors are in addition to the senior conductors and have different duties. This is about safety – as we have told the RMT many times, we have no intention of removing conductors from our trains.”
The union has also reaffirmed its Southern rail walkout on Monday 23 January, after saying it had been "excluded" from talks between the train operator and train drivers' union Aslef.
Southern said it will be able to run more than 70 per cent of its trains on Monday and hopes to bring back a full service from Tuesday.
It comes as Southern talks with Aslef enter a third day. The union suspended its own strikes planned for 24, 25 and 27 January and its overtime ban in exchange for fresh talks.
A Southern spokesperson said the company was "happy to have formal talks with the RMT too" and "lines of communication with them are open".