The 4000-series railcars, which were part of the Washington Metropolitan Area Transit Authority (WMATA) fleet, faced restrictions from being used in leading positions primarily due to safety and performance concerns. Here are some key reasons for this decision:

1. Structural Integrity: Tests revealed concerns about the crashworthiness of the railcars. The 4000-series did not meet the robust safety standards required for leading railcars, especially in the event of a collision.

2. Mechanical and Electrical Reliability: There were issues related to the mechanical and electrical systems of the 4000-series cars. This included reports of frequent breakdowns and performance inconsistencies that were not suitable for railcars leading a train.

3. Safety Standards: As newer and more advanced railcars, like the 7000-series, were introduced, they were equipped with better safety features and technology, making the older 4000-series less preferable in leading positions.

Due to these factors, WMATA decided to limit the use of 4000-series railcars in positions that would impact the safety and efficiency of the trains. For more in-depth information, you can refer to official WMATA publications or news articles discussing their operational strategies and safety upgrades.

If you're interested in further details or a specific aspect of the railcars' operation, feel free to ask!