The PATH rail system may not install a new crash-prevention system by December 2015 after all, a person involved in the project said, even though the federal deadline was one reason that officials gave just weeks ago for shutting down weekend service between Jersey City and the World Trade Center for a year.

In announcing the work on the World Trade Center tunnels last month, PATH officials said they could meet the deadline, which would make it one of the few commuter railroads in the country to do so. Installing the system,...