DC systems (especially third-rail systems) are limited to relatively low voltages and this can limit the size and speed of trains and cannot use low-level platform and also limit the amount of air-conditioning that the trains can provide. This may be a factor favouring overhead wires and high-voltage AC, even for urban usage. In practice, the top speed of trains on third-rail systems is limited to 100 mph (160 km/h) because above that speed reliable <answer>contact between the shoe and the rail</answer> cannot be maintained.
What can become unreliable if the speed exceeds the limit of 100 mph for DC system trains?
contact between the shoe and the rail