Certainly, as vehicles can’t overtake other slower vehicles, the only way to get full speed is to replace all old vehicles in a line with new ones.
Regarding your second question, in the window of whatever line, there is a tab that offers you the possibility of automatic vehicle replacement. You can set the vehicle that will replace the old one and at what time (how old has the vehicle to be in order to be replaced), but note that there are several shortcomings:
- The new vehicle will be automatically replaced when the old one reaches the following stop, but unfortunately all cargo/passengers will be lost. And those lost cargo/passengers will negatively count for the line/industry statistics.
- If the vehicle you have set for replacement or any part of it becomes obsolete with the pass of time, automatic replacement won’t take place any longer and you won’t be notified.