Rent a pipe scoring tool.

It is a chain with hard wheels on the links that goes around the pipe and scores the surface.
The cast iron is then snapped off on the score line.

Sewer gas is flammable, so tolls that produce sparks (angle grinder) or can result in heating the material in the cut (sawzall) are not recommended unless the pipe is no longer connected to a sewer system.
As long as it is connected to a system it can have flammable gases present in the pipe.

Even if it's a shower drain?

It does not matter what th eiron pipe is being used for.

You do need to be sure it is iron pipe and not galvanized steel pipe.

Iron pipe was not used all that much in size below 3 inches.

Old iron pipe has belled ends (AKA 'hubs') with joints caulked (oakum) and topped with lead (the lead is poured over the caulk and then used to compress the caulk to create the seal).

Some newer iron pipe has no hubs and uses metal jacketed elastomeric clamps to join section (called 'no hub').

Galvanized steel can be cut wit a typical 'pipe cutter' with a hardened wheel.

Thanks Brickeyee, I'm pretty sure this is cast. The pipe is very thick walled, not like any galvanized I've ever seen and given the age of the house, odds are... I'll try the scoring tool and see how it goes.
How far do I score it and what do I use to snap it off? Thanks again!

Is there any access to the pipe from a basement or crawl space below the floor?

If so, the easiest method is to go under the floor and cut the line back a couple feet using s chain snap cutter to cut the cast, the use a Fernco to conect either PVC or ABS and install a new closet bend up to the bathroom