 Caution must be exercised by the leading train not to release authority on track that the train still occupies, as this could result in a rear-end collision if the leading train slows down or stops or if the train becomes separated and a portion is left standing on the main track.Entering a block in which the train is not authorized or releasing a block which the train still occupies is considered a main-line violation by the Federal Railroad Administration and carries stiff penalties for all crew members, which may include time off from work and the suspension or revocation of the engineer's license.The Southern Pacific Railroad, now the Union Pacific, implemented DTC on several of its lines formerly controlled by train orders (most of the rest of UP's lines use Centralized Traffic Control).