Which of the following could NOT be a cause for a train running slow?

High third rail voltage does not typically cause a train to run slowly; rather, it usually indicates that the system is functioning properly. The third rail provides the necessary electrical power to the train's motors, and a high voltage reading often suggests that there is sufficient power available for optimal operation.

A train running slowly could stem from issues like low air pressure, which affects the braking and performance systems, or from cylinders failing to engage, leading to a lack of necessary traction or propulsion. Additionally, a heavy cargo load can impede acceleration and lead to slower speeds due to weight constraints. These factors directly impact the operation of train systems and can be significant contributors to reduced speed, contrary to the effect of voltage levels on the train's performance.