When is it acceptable to increase speed if governed by a signal displaying Limited Clear?

The correct answer is that it is acceptable to increase speed once the entire train has passed spring switches when governed by a signal displaying Limited Clear. This means that train crews must ensure that all components of the train, particularly the rear, are clear of any potential hazards, such as switches that could affect train movement, before increasing speed.

Limited Clear signals typically indicate that the train can proceed but must be cautious, as there may be restrictions or upcoming conditions that require the train to be prepared to stop. Therefore, maintaining awareness of the train's position relative to switches is critical for safe operation. By waiting until the entire train has passed the spring switches, crews ensure that no part of the train interferes with switch mechanisms, thus preventing potential derailments or other incidents associated with improper clearances.

Other choices are not appropriate due to safety protocol considerations. For instance, increasing speed immediately on crew's discretion disregards the necessary clearance checks and could lead to risks. Similarly, waiting until just the first car is clear of switches or until reaching the next station does not ensure that all parts of the train are clear, making them inadequate safety practices.