First and foremost, the railways of Queensland as opposed to Queensland Railways is certainly a mixed bag when it comes to radio, but AM is definitely not part of the mix.
QR in the south east and one or two other areas is using Tier III DMR for its operations. Analog FM is still used as well but is slowly being phased out.
Aurizon has just rolled out a TETRA platform for train control etc on lines under its control. Again some FM can still be found but it too is being replaced by TETRA. For most part TETRA is largely unscannerable (if that is a word). Only one receiver can monitor TETRA but cannot track it or display talkgroups and associated network information. SDR devices and computers can also be used but a limiting. TETRA uses four timeslots within a 25 kHz channel, each timeslot is 6.25 kHz wide and all four can be used simultaneously.
Back to the QR DMR and DMR in general. Forget about Tier I, it is basically used for kids toys in Europe and Asia and is effectively useless. Tier II is conventional while Tier III is trunking. QR is trunking. Trunking is basically described as using a pool of frequencies at each site, whereby a user is assigned a frequency from the pool, after which is returns to the pool for the next subscriber. Sites are normally linked by various methods including but not limited to RF or IP connections.
DMR employs two timeslots of 6.25 kHz each which makes up a standard 12.5 kHz RF assignment. These timeslots form virtual channels. The timeslots can be assigned specific functions like voice only, data only, voice and data, telemetry and GPS to mention but a few. Both timeslots can be used simultaneously.
It appears from your post that you are somewhat of a novice when it comes to the newer digital formats and the new breed of scanner. Without meaning any offence if that is the case then don't purchase a USDS100, instead look at a good used UBCD436PT or the US version BCD436HP. Firstly they will cost you significantly less but can still do the same job. Maybe they don't have all the pretty lights but you do pay a premium for those lights.
The 436 can do DMR (paid upgrade) just as well as the USDS100. This will get you started and if you decide to continue then upgrade to a USDS100 can follow. You can sell or keep the 436 as a backup. Ebay is a good source for the 436's often with the DMR upgrade already installed.
There is no easy way I can provide enough information in this forum to get you started in monitoring QR DMR. I would be typing for a month of Sundays. Again without offending, you are going to start a journey on a very steep learning curve. I can only say to learn as much as you can about the scanner and the systems you want to monitor and don't look to others to spoon feed you. While that may sound tough, learning yourself means that can work things out when something goes wrong or changes. Besides information on forums like this and others is often wrong or inaccurate. Present company excluded, of course.
Post here your decision.