Context: 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.

Question: Why DC system can effect the speed of trains?
Answer: low voltages

Question: Can DC systems use low-level platform?
Answer: cannot

Question: Besides speed and size what else the low voltage of DC system can effect?
Answer: air-conditioning

Question: What is the speed limit for third rail system trains?
Answer: 160 km/h

Question: What can become unreliable if the speed exceeds the limit of 100 mph for DC system trains?
Answer: contact between the shoe and the rail

Question: DC systems are limited to relatively high what?
Answer: Unanswerable

Question: The lowest speed of trains on third-rail systems is limited to what?
Answer: Unanswerable

Question: Above what speed can reliable contact between the shoe and the rail be maintained?
Answer: Unanswerable

Question: What can not limit the size and speed of trains?
Answer: Unanswerable

Question: What is not a factor favoring overhead wires and high-voltage AC?
Answer: Unanswerable