Clone of the built-in Portal with the following additions:
* Option to produce the first train immediately (instead of waiting for the delay time to elapse).
* Option to produce trains after a random time between min and max. To use a fixed interval, set min and max the same.
* Option to regurgitate trains after a random delay between min and max. To use a fixed delay, set min and max the same.
Using random intervals/delays inject some variance into scenarios.
Note that buildings cannot be replaced. Thus it is not possible to update an existing Portal into a Portal Immediate Random.