 Another factor favoring standard gauge is that low-floor vehicles are becoming popular, and there is generally insufficient space for wheelchairs to move between the wheels in a narrow gauge layout.The use of a proof-of-payment fare collection system is standard practice on modern North American light rail systems, as it reduces the dwell time at stations by allowing passengers to board and alight at all doorways of a light rail vehicle (LRV) or train.A few recently opened systems in North America use diesel multiple units, including