We have a house tat was badly damaged in Hurricane Sandy, and needed to be gutted down to the studs. We are using this opportunity to change the layout of this 800 sf, one bath house to work better for us.It is on a slab. Part of this is trying to fit an extra half bath. The layout such that the exising toilet drain could be in the new half bath and existing shower drain would be in the new full bath requiring a new waste line for the new toiilet and sink. My question is, is the same waste line used for all parts of the bathroom, shower, toilet . Ex can a shower drain also be used as a toilet drain? Or are the different.

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My question is the same waste line used for all parts of the bathroom, shower, toilet . Ex c

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Yes. A 3 or 4" main drain coming from the toilet will have branches in the main drain to pick up the tub/shower and lavatory, The bathroom vent will come off the lavatory and vent the entire group, (see image).

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can a shower drain also be used as a toilet drain?

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No. A shower drain's too small for a toilet. Drain. More questions? I'm as close as a click. Tom

Sink and shower drains can possibly be swapped but are 1 1/2 or 2", a toilet drain is 3 or 4" . Special grinding toilets can be installed for smaller lines but are much more expensive. You could possibly install back to back rear flushing toilets if you layout could possibly be changed. You are facing typical slab house problems. Most likely the only solution is to cut the floor for the plumbing changes.