On Screen Barcodes
***"Existing mobile payment systems, such as the bar codes being deployed by Masabi on the UK train networks, can't be read by another handset as phone cameras aren't good enough to read a code displayed on-screen ... at least not yet."***
That certainly isn't my experience, though it can be rather hit and miss, which wouldn't be much use at a busy ticket barrier.
However, there are plenty of Bluetooth "ring" scanners (1D and 2D) that can be coupled to the phone. These cost about £500 each, but still pretty cheap compared to a full POS terminal.