It is a while since I built or rebuilt my layout, my memory is not what it was.

A question about using multiple transformers. If I remember anything incorrectly please tell me.

Obviously it is important to make sure that the polarity of the transformers is the same. This I remember. It is not difficult to check with UK plugs.

The positive (red) should be kept separate.

The negative (brown) can be connected at the transformers

This is the one I am not sure about:

The accessory power (yellow) can be connected at the transformers or should it be kept separate for each track or is it simply a question of load?

As PacoM states...

Brown (O) can be considered as 'ground' or 'earth', (from the German "Masse") or more what we might think of as 'common' so yes the brown outputs can be safely joined together.

Red (B) and Yellow (L) on the other hand must not be mixed with the same outputs of other Trafo's, the specific difference is that Red is a Controlled output, matching the Red knob, while Yellow is uncontrolled.

So long as the power requirements don't exceed the rated output of each Trafo, you should have reliable operation.

Hope this helps

Cookee

In order to make it clearer, is your track “M” type or not (C or K)? In any case, if there are two transformers that govern each the train in one section of track, it is indifferent whether the accesories belong to one section or the other, whether they are supplied by one transformer or the other or some by one transformer and and others by the other.

Yes
Brown join is fine; common ground.
Red join not. Track power (segmented sections).
Yellow join not. Accessories power (anywhere).
Use new white transformers and ditch old blues; latter are not protected well for shorts and overload.

This can be true in some cases but not all.

Blue Trafo's with Plastic cases are generally fine, and even more so if being used with Loco's and accessories from the same era or older.

The latest series of Blue Trafo's with METAL cases 'may' be ok but the PVC mains cord should be checked regularly for deterioration.

Older Blue Metal Trafo's from the 1950's (280A style) have well-known issues with the rubber insulation perishing on the mains cords, these MUST be replaced with a modern cord and preferably by a qualified appliance or electrical technician who can also do the safety tests.

Generally, although the cords fail, the rest of the unit is usually in good condition internally and again, fine to use with Loco's and Accessories of the same era.

Quite apart from that, replacing multiple Transformers (unnecessarily) could be an expensive exercise. Safety of course is not measured in $ & c but some caution and common sense will win out.

Cheers

As stated, BROWN should be connected together among all trafos.

Regarding the polarity; Dont trust the wall-plugs, but DO the actual test:
Once BROWN has been connected between 2 trafos, set the speed-knob to "100" on both of them. Grab a 16-24V light bulb, and connect one wire to RED on trafo 1 and the other wire to RED on trafo 2. The bulb should remain dark!
If the bulb lights up, then one of the trafos wall-plugs must be TURNED 180º.
-If your british-type wall-plugs doesnt allow that, the plug must be remade by cutting the cable and re-connecting the wires.

Once BROWN is shared on all trafos, the accessories can be freely connected to the yellow connector on any of the trafos, as long as the total power consumption doesnt surpass the trafos rating; If you use the trafo for running trains, you might calculate around 10VA for each loco, plus 1VA for each bulp (lights, signals, interior passenger coach lighting).
Old-style coach-lighting can really add up, with 2 bulps per coach!
Solenoid actuated accessories consumes around 3VA, but only during their actual activation, and ususally they are only activated one at a time.
Older blue plastic trafos can give around 30VA and the newer white ones around 32VA.
(Older blue ones are great for powering lights and accesories!)

For dedicated lights transformer, you can also use red (instead of yellow) and brown, allowing you to dim them (variable voltage).

That is why yellow outputs should never be connected together