Under what condition can a train start movement in Release mode?

The correct understanding is that a train can start movement in Release mode primarily when it is within 50 feet of another train in Communication-Based Train Control (CBTC) mainline territory. This condition is critical as it signifies the need for close monitoring and coordination between trains to ensure safe operations. In CBTC systems, trains can communicate their positions and statuses to maintain proper spacing and safe movement, especially when they are in close proximity to one another.

The other conditions listed do not necessitate or guarantee the ability for a train to safely enter Release mode. For instance, being on an uphill grade or having a full charge does not adequately address the safety protocols necessary for train movement in Release mode. Similarly, knowing the destination alone does not provide the necessary safeguards that CBTC systems are designed to enforce when it comes to train interactions and operations in the system.