Context: Railways must operate at variable speeds. Until the mid 1980s this was only practical with the brush-type DC motor, although such DC can be supplied from an AC catenary via on-board electric power conversion. Since such conversion was not well developed in the late 19th century and early 20th century, most early electrified railways used DC and many still do, particularly rapid transit (subways) and trams. Speed was controlled by connecting the traction motors in various series-parallel combinations, by varying the traction motors' fields, and by inserting and removing starting resistances to limit motor current.

Question: What is speed limit for railways?
Answer: variable speeds

Question: Why is DC motor being used more than AC type?
Answer: conversion was not well developed

Question: What are two types of railway transportation still use DC motor?
Answer: rapid transit (subways) and trams

Question: What must operate at constant speeds?
Answer: Unanswerable

Question: Speed was controlled by disconnecting the traction motors where?
Answer: Unanswerable

Question: A brush-type BC motor operated at what speeds?
Answer: Unanswerable

Question: All early electrified motors used what type of motor?
Answer: Unanswerable

Question: Varying the transition motors' fields helped control what?
Answer: Unanswerable