As an example, in the case of where wiring terminates to a solder post, some mechanical retention must also be used. Like hooking or wrapping the wire, not just soldering (I think Phil mentioned this already). A barrier isn't required unless the spearation distance has to be reduced - in which case the separating distance becomes the distance around the barrier.
Insulated mains wiring lying over insulated secondary wiring would be OK if both lots of insulation was rated for the highest working voltage. Most PVC insulation used for the last twenty years is rated to at least 300V, including extra low voltage wiring (its usually written on it). After all, its common to see mains and elv wiring in the same loom.
If the transformer frame is separated from 'safe' parts by 3.0 mm distance or insulation as per the above, then its probably OK.
Once the covers are off it is up to the 'trained personel' to apply their knowledge and determine what areas are safe. Its not unreasonable to expect trained people to exercise due care. There is a difference between ignorance and negligence.
There are legal tests for 'reasonable' but that could fill an entire forum all on its own. However here are brief examples:
If a device has a mains cord that terminates in the equipment, it is reasonable to expect the device to contain electrical hazards and for a trained person to know this.
But, if a device is run from +24V DC, and has an internal inverter producing 110V AC and 300V DC for use only inside the device, then it may be unreasonable to expect a trained person to know where the hazards are (UNLESS the trained person has had specific training on that apparatus).
In the latter example, the internal hazardous parts may be labelled or guarded to reduce the risk to service personel.
The internal parts are not accessible to users and hazardous voltage enters the equipment. A trained person would be aware that a danger exists and through a combination of examination and measurement determines areas that are safe. If you need tools to open it, and users are not instructed to enter it, no warning is needed.