They use rail to get the oil Mereenie produces to Adelaide...

They use rail to get the oil Mereenie produces to Adelaide already. Trucks and rail are effective for oil because it's a liquid, though pipelines are usually a better option if the oil deposit is big enough.

You can't really use rail to shift gas. You would need to liquefy it first, or at the very least compress it. Neither are cost effective compared with pipelines which is why they use pipelines.

A few companies have half-heartedly proposed mini-LNG or compressed gas projects in Central Australia to allow them to monetise small deposits that aren't big enough to justify pipelines. I think even CTP might have mentioned it at some stage in the last decade. But I don't think any of them were ever seriously advanced because the economics don't stack up.