Rail operators that failed to install the system by Dec. 31 would have faced hefty fines and legal liabilities. But as part of a short-term highway funding extension approved by the House on Tuesday and the Senate on Wednesday, the new deadline would be end of 2018, with the possibility of further extensions for some operators.

The original deadline for the so-called “Positive Train Control” was set by the Rail Safety Improvement Act of 2008. Rail industry representatives called the deadline arbitrary because the technology wasn’t commercially available at the time— and, therefore, no one really knew when it would be ready.

Though railways have reported spending billions of dollars on implementing the new safety system, it has been clear for nearly two years would be unable to meet the original deadline because of technical challenges and the vast American rail network. The slow rollout became a topic of hot debate earlier this year when an Amtrak train derailed in Philadelphia on a section of track without the safety system installed, killing eight people.

Railroads and passenger systems have spent recent months threatening service shutdowns as soon as the end of October, and issuing dire warnings about the potential impact of a broader shutdown. Earlier this month, in a conference call with several audibly frustrated rail executives, Metra CEO Donald Orseno told reporters the Chicago passenger rail system would have to be shut down without a deadline extension.

On the freight side, the American Chemistry Council–which represents hazardous material producers that would have been particularly impacted – concluded that curtailed rail service could have triggered a 2.6% GDP contraction starting in the first quarter of next year.

In normal political times, these declarations and reports might have seemed like a bit of kabuki – there’s no way that Congress could let America’s trains shut down because of a deadline they imposed themselves, right? But considering that legislators’ inability to reliably fund highways has already become a perennial drain on the national economy, perhaps worst-case scenarios were worth talking about.

That’s especially true because there was real opposition to the extension, particularly from Sen. Barbara Boxer (D-Calif.), who had instead proposed a much stricter system of case-by-case, one-year extensions. Boxer reportedly dropped her opposition in exchange for confirmation of new Federal Railroad Administration head Sarah Feinberg.

President Obama is expected to sign the highway bill and accompanying smart train extension soon.

For more about trains, watch this Fortune video: