Amtrak’s $2 billion effort to replace its fleet of Acela express trains on the Washington-to-Boston corridor has hit a foreseeable snag: The new trains have to run on an old railroad.

The trains from France-based Alstom were originally set to begin service in 2021, but Amtrak and Alstom officials now say they will remain sidetracked at least until 2024 as the manufacturer struggles with mandatory testing intended to show how the units will perform in real-world conditions.

The core problem, they say, is that the Acela runs on tracks shared with commuter and freight trains—not the purpose-built, high-speed tracks found in Europe and Asia

The company says the articulated design of the new units will provide a smoother ride, and that faster acceleration will speed up trip times, even though the new trains will only be 10 miles-an-hour faster than the current units at top speed.

Amtrak officials acknowledged their frustration with the delays in bringing the new trains into service, through one of its largest-ever procurements and an upgrade to one of the company’s most important brands.