There are two critical safety issues that affect passengers using passenger rail platforms: platform-to-railcar horizontal gap and platform-to-railcar vertical gap. Rail passenger gap falls occur when an object or the nature of the walking surface prevents or delays the passenger’s rear leg from moving forward to achieve a safe footing at the instant the foot makes contact with the surface ahead.  Safe walking requires perfect timing in the transfer of support and balance from one leg to the other.  The slightest change can result in an imbalance that can result in a serious fall as the body continues to move forward.  There are two aspects to the platform gap: the vertical difference between the car floor and platform surface elevation and the horizontal separation of the railcar from the platform.  Today’s rail transit equipment uses mechanical and automatic car floor leveling systems to maintain the car floor nearly level with the passenger platform.  This is an important safety feature because when the railcar floor is above the platform, it creates a tripping hazard for passengers boarding the vehicle; when the car floor is below the passenger platform, it is a tripping hazard for passengers exiting the railcar.  Most falls are initially precipitated by a trip, and if the gap is wide enough, extension of the passenger’s leg might fall into the space.  Most modern rail systems, with proper design and construction of platforms, together with proper maintenance and proper car floor leveling devices, can attain safe vertical and horizontal gap clearances.  This makes it unlikely that a passenger will trip on the car doorsill or platform, or that a passenger’s foot will pass between the car and the platform.