In Trainz, there's no range. Once you pass it, that's the speed forever.
Depending on the Railroad, but also generally no. The speed limit is set and then can stay that for 100 miles, but generally there'll be another sign somewhere along that route.
There are exceptions though - in my state in Australia the driver's were tasked with knowing the max speed for the line, and then curveboards were used, slowing down around corners with no speed signs on the way out - the driver's just sped back up when they had passed the curve.
Cheers
Jamie