Train Arrival times

Hi,

I have 2 metros in my model, lets say A and B, A has much smaller alighting than train B but has a lot of boarding passengers. Both metros has a headway of 90 sec. What is happening is because of few alighting figures in metro A, it is Arriving at a larger headway than 90 sec i.e train not arriving every 90 sec , What do I do to make the train to arrive every 90 sec?