I believe the railroad industry (the Association of American Railroads) has been saying all along that they wanted the final deadline pushed back to December 2020.
Railroads will have the option of filing an alternative schedule for implementation with the US DOT. An alternative schedule must outline how a railroad will implement PTC no later than December 31, 2020, and will be subject to the binding review and approval of the US Secretary of Transportation. Starting in March 2016, railroads must file detailed annual PTC progress reports to US DOT until PTC is fully installed on each railroad's network. The US Secretary of Transportation will assess civil penalties to railroads that do not comply with the law. Link to AAR
I think the difference is, the systems will have to be fully designed and should be implemented by 2018 but testing can continue over two more years. There will be a lot of testing done before the systems will become fully operational. In addition, railroads who still can not meet the 2018 deadline will have two more years in which to do so. But they have to have already filed a detailed report with US DOT outlining their schedule.
Below is a chart showing industry progress through this past August as well as a timeline for the rest of the work.