Just a few examples of things that I have found amiss in the course of the preps that I've carried out in my driving career:
- Horn left isolated after maintenance. You certainly wouldn't want to wait until you're trying to sound a warning to trackworkers or crossing users to find that out!
- Isolating cocks below solebar left isolated. I'm aware of at least one example - probably more - of a train (presumably previously held by scotches) being moved with the brakes isolated (once on the move, it kept going). It was only a shunt move around a maintenance depot, but the potential is there still.
- Defective safety systems. On our trains, at least, they (TPWS excepted) don't self-test.
- Engine oil or transmission oil just about to disappear out of the very bottom of the sight glass. It's maybe not immediately dangerous, but it'll most likely lead to a failure situation later in the day, which is potentially very inconvenient and also carries risks of its own (the first you know of low oil pressure on our trains is when the affected engine shuts down, and even then you have to go looking for the problem).
- Things amiss in the rear cab, some of which could immediately affect the safety of the train and some of which would merely be an inconvenience when I or the next driver came to change ends and found the defect, potentially failing the train in an inconvenient location.
Could there be more status lights in the cab? Put them behind the driver so it’s not a distraction. A simple red yellow green status light on oil, does that need checking more than once a shift?
When I take a hire bike I briefly test it out before even unlocking it. Sometimes the same tests with my own bike, stored in my own garage. There’s no need for complacency.
As others have said, newer trains will often flag issues up through the TMS, but there are plenty of trains out there that don't have much in the way of electronic sensors, nor any means of transmitting anything other than a couple of general fault indications to the leading cab from the affected vehicle. Generally trains are only prepped once per day anyway, one exception being at some companies where they have to be prepped again if they're left shut down for a period of time during the day (e.g. between the peaks).