At the station visible at opening, I have two terminating lines: one has 12 trains per month with a 2:30 offset and the other 36 tpm with a 5:00 offset, yet trains of the less frequent line still depart simultaneously with a train of the more frequent one.
Save: https://www.dropbox.com/s/zht76xtk2snlejz/Britain%202.sve?dl=0
I can't seem to open the save, but here's a comparison of departure times of your choice:
Line 1; 0:2:30; 0:15:18; 0:28:06; 0:40:54; 0:53:42; 1:06:30; 1:19:18
Line 2; 0:5:00; 0:15:40; 0:26:20; 0:37:00; 0:47:40; 0:58:20; 1:09:00; 1:19:40
What versions of executable and pak are you running?
I'm playing the (nearly) latest development versions of Simutrans and pak128.Britain-Ex with bits_per_month=22 and meters_per_tile=117, so I have a month duration of 5:59:25, which gives me approx. a train every 30 min for one line and every 10 min for the other. Incidentally, I'll also correct the first post.
Can you identify which specific schedules depart simultaneously and when they should, in fact, depart?