You'd be surprised, but it's actually not like that for the most part. Some shifts ("Local Relief" or "Standby" types) yes, but they're the exception rather than the rule.
The job itself is not "hard" per se' and you can be re-tasked if your originally-assigned train is cancelled, but
"forecast rosters" are something that has been (
very) hard fought for in various depots over time and they are a
major selling point for companies in attracting and
retaining crew. Ie Your Master Roster has you doing Shift pattern "Central-05A"and the
actual working of "Central-05A" may be
"All stations to Penriff, then All stations to North-Sydney & have crib. Pick up next train at North Sydney and work all stations to Berowra then all stations to Central & signoff at time xx:xx." You know your signon time, your distance travelled (as you can get km-based payments if you work over X km's) and your signoff time down to the minute (if everything works normally). An
actual Sydney Trains job diagram is shown in this video at the 15s mark. Apparently I can't just link it, it shows the actual preview instead of the BBcode text...
https://www.youtube.com/watch?v=poxoz4MIilA&t=15s
Some depots have what is known as
"Blank line" or
"Pencil" working, where you know your RDO's and that is it. You don't know what you're doing tomorrow until you sign off
today and the working is almost exactly as you describe. ie my old signoff phone call would be "Cheers, KR, got you off at 1600, job for tomorrow is 0300 on, work train 1535 to Walgett and load." Other depots have specific rostered sign-on times(and usually signoff as well, when things are running well) for the typical working of that depot (usually container & passenger trains) as they run to a timetable. Blank line working, though, is shyte. You can't plan a damn thing, fatigue is a major problem and consequently so is retention, so some companies actively market their forecast rosters in their job ads,
like PN do here.
The ST problem they have created for themselves through lack of recruitment, is twofold. One is the ongoing requirement to have crews work on their RDO's
just to run a 'normal' timetable and the other is the constant changing of roster's (with the attendant changes in signon/signoff times) to try to cover the normal timetable (or services they deem a priority) without sufficient crew to do so.