The Sentinel is a between car barrier designed to enhance the safety of multicar high speed rail trains that utilize elevated platforms with level boarding. It creates a barrier to prevent, deter and warn passengers, onlookers, workers, and others in the vicinity of the train (especially the vision impaired) from accidentally entering the space between the cars while the train is at rest, thereby reducing the risk of injury.

On October 1, 2007, The Secretary of Transportation issued a directive under Title 49, Subtitle A, Part 38-Americans with Disabilities Act (ADA) Accessibility Specifications for Transportation Vehicles, subpart d-Light Rail Vehicles andSystems, Section 38.85 Between Car Barriers that states:

"Where vehicles operate in a high-platform, level-boarding modes, devices or systems shall be provided to prevent, deter, or warn individuals from inadvertently stepping off the platform between cars"