generate questions: 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 contact between the shoe and the rail cannot be maintained.
Why DC system can effect the speed of trains? {sep_token} Can DC systems use low-level platform? {sep_token} Besides speed and size what else the low voltage of DC system can effect? {sep_token} What is the speed limit for third rail system trains? {sep_token} What can become unreliable if the speed exceeds the limit of 100 mph for DC system trains? {sep_token}