Let's say that I want to create a station platform for TRS2004. The bottom of the platform is on the ground grid plane. The default position of the pivot point is in the center of a group of objects. How do I move the pivot point to the bottom precisely?

A few options...

If you have a single object, select 2 diagonal points on the base (or the lower face of the base if you've not already deleted it) and run my ShiftCentre plugin (or operation)

There is also a ShiftGroup script on my website that lets you enter a value to shift a group axis.

Lastly, you can do it by eye - select the Shift tool on the right, hold down the Ctrl Key and Right mouse drag the Axis down to the floor level.