Most Class 1 road locomotives are equipped with various GPS technologies, such as WiTronix, in preparation for PTC implementation. These systems give supervisors the tools to watch (or download) the locomotive in real-time (or download past events on demand much more rapidly and easily than the event recorder days).

WiTronix specifically can even be programmed to trigger alerts to management when certain operating parameters or thresholds are reached or exceeded, whether it be speed, air brake use, dynamic brake use, independent brake use, excessive running idle time, etc. At least some Class 1 railroads (BNSF and CN I know for sure) both have staff in place at there headquarters (BNSF calls them "remote auditors") to periodically watch these new data streams on random trains throughout the day to ensure compliance with both operating rules (which would include speed limitations) and air brake/train handling rules (proper brake utilization, etc.).

So yes, this is watch much closer today than even a few years ago. That is not to say that a train crew could not still inadvertently exceed a speed limit without getting caught, but the frequency of violations that slip through the cracks undetected are becoming fewer and farther between as technology progresses.