NEWARK, N.J. (AP) — At least four New Jersey Transit trains were cancelled Wednesday morning, snarling the commute for rail riders.

By 7:30 a.m., one train each had been cancelled on the Raritan Valley, Montclair Boonton and Pascack Valley lines. Two had been cancelled on the Morris and Essex line.

NJ Transit said Wednesday that several factors are contributing to recent train cancellations.

The installation of a federally mandated emergency braking system is requiring train crews to perform testing, causing higher than usual crew absences.

Additional locomotive engineers are being recruited and trained to address the shortages.

NJ Transit recently received approval for a two-year extension to install the system, beyond the initial deadline of Dec. 31, 2018.