having the same problem, but i noticed something. watching the affected line for a few minutes, it would constantly glitch, showing one vehicle being added for a split second before going back to zero. it's usually also subtracted from another line. it's weird

That because you didn't make a direct connection to the first station.
It need direct track to the first station that you lay that route path down, it won't spawn any vehicle if you only connect to those second station.