What condition must be met for movement at restricted speed?

For movement at restricted speed, the essential requirement is that the train must be able to stop within half the range of vision. This condition ensures safety by allowing the crew to react to any obstacles or signals they might encounter on the track. The concept of restricted speed is critical in scenarios where visibility may be limited, or the track is not fully clear, such as in areas with heavy rail traffic, approaching switches, or during low-light conditions. By adhering to this guideline, train operators can minimize the risk of collisions and ensure a higher level of safety for both passengers and crew.

The other choices do not align with the requirements for restricted speed. Maintaining a specific speed like 30 MPH is not applicable because restricted speed is primarily about the stopping distance rather than maintaining a set speed. Likewise, while complying with posted speed limits is important in general rail operations, it doesn't specifically address the unique requirements of restricted speed. Lastly, ensuring a green signal is not pertinent to restricted speed conditions, as trains may encounter situations that require this speed even with signals not being green. Thus, the focus is on the ability to stop safely within a limited distance based on visibility.