In response to these issues, low-floor light rail vehicles were developed. In a low-floor car, either the middle portion or all the vehicle floor is positioned a short distance above top of rail. A typical dimension is 300 to 350 mm [about 11.7 ¾ to 13 ¾ inches]. This enables station platforms to be little more than sidewalks that are just slightly higher than normal above the street surface, making them much more practical for construction in congested urban areas.
Since about 1995, the partial low-floor car (often called a “70% low-floor” LRV) has become the preferred design for North American light rail transit systems that need level boarding from low platforms. The partial low-floor car has some middle portion of the LRV at the lower elevation while the ends of the car are at normal high-floor car elevation. The doors are usually all in the low-floor section of the car and the high-floor areas at the ends of the car are accessed by interior steps.
The low-floor area usually represents approximately 70% of the total length of the car, hence the common name. (Boston’s Type 8 LRVs are a notable exception; clearance limitations in the Green Line tunnels substantially restricted the truck center distance so that the low-floor portion of each car is only about 60% of the overall length.)
One advantage of a 100% low-floor LRV is that the low profile of the cab and windshield increases the probability of eye contact between the operator and persons on the trackside. A corresponding advantage to a high-floor or 70% low-floor LRV is that the operator’s higher seating provides a better view of the trackway ahead, which could be an advantage in some traffic situations.
One possible issue with low-floor cars is that they maintain very close clearance to rails. With worn-out wheels, the vertical clearance between the underside of truck-mounted equipment and the plane of the top of rail can be a little as 35 mm [1 3/8 inches]. This could affect the use of some trackwork and signal system appliances mounted between the rails. The vehicle clearance also must be considered in design of tracks for hilly terrain, where the radius of the vertical curve over the crown of the street must be large. On one project, the low underclearance of the vehicle limited the height of discontinuous floating slabs that could be used, where maximum mass is needed for vibration control.
