I am a keen cyclist and like to take my bike me when I go away for more than a week or so. As such, my bikes ended up on a train with me from Melbourne to Sydney and I struggled to find the information I needed. I hope this helps!
Booking
At this time you need to call NSW trains to add a bike to your booking. It costs around $12 in addition to your normal ticket and they’ll issue you a new ticket with that info on. I booked my main ticket online, then called to book my bike.
Dimensions
The dimensions for the bike to travel are TINY. I couldn’t find a commercial bag or box to fix this for love more more, until I stumbled across the NZ brand Groundeffect. Their Tardis bag is the only one I believe that is the correct dimensions.
You can get a box from the station and you can also collect this in advance so you’re not breaking your bike down in the station. My challenge here was in cannot carry a box with my other stuff and, as I travel a lot, I figured buying a bag would be a good option. I was Lucky enough to find a second hand ground effect bag and I’m very happy with it.

Bag or box?
While on the phone to NSW trains they were adamant I had to use a box, but when I actually handed my bike over in its bag, they took it as is. I think it really depends on who you get at the desk on the day. I’ve seen other people say on Reddit that they had to put theirs into a box from their commercial bag.
The beauty of the tardis is it does, in theory, fit inside the box they give you anyway so you’re not struggling on the day either way. I can’t confirm this yet but I probably will be able to soon!
Checking in
It’s like an airport bag drop with a. Counter and belt- they weight the bike then send it through, giving you a luggage ticket in return. I understand on the train it’s stored upright but on the carousel it was laid flat so do bear this in mind when packing.
I was at drop off around an hour before (far too early in reality, but as my husband can attest to I am a panic traveller and always think I’m going to miss my train/boat/flight. I wish I wasn’t, but I am!). I’d recommend aiming for at least 10 minutes before bag drop closes in case of issues.

How did it travel?
My bike arrived unscathed, with no dings or otherwise. I had put some padding in there but not a wild amount. My current bike has an aluminium frame – not the best in terms of weight but very hardy (a calculated choice given how much I travel).
Collecting
When I got off the train in Sydney there was a pile of bags being unloaded. I’saw my bike coming off and handed over the tag in exchange for the bike. I would recommend getting off the train sharpish to retrieve your bike as I think mine could possibly have ended up lying on the platform had I not been there to retrieve it. No big issues, just not ideal.
Other options
If you want to travel your bike whole, i.e. not broken down, Firefly and Greyhound are options. I found when I spoke to them Firefly are easier to deal with but they don’t have the same reach as Greyhound so it depends where you’re going. Having taken a firefly bus, I’d recommend being early to check in if you have a bike as people do seem to have a lot of stuff on those buses.