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They can be run together but there is no positive locking under tension (as the mark 3's name suggests!). What this means is that if there is any uneven track the couplings may possibly disengage. The mark 2s cannot be run forwards over a springy uncoupling ramp intended for the mark 3s as the hooks will simply spring up and disengage.

If you are prepared to do a bit of work on the mark 2s they can be modified to allow more positive engagement - that depends on whether you are a collector and wish to keep everything mint and as it left the factory or not. I have done it to a couple of old utility vans and a southern electric unit and the method is as follows:

1. You have to grind or file a shallower depth into the crossbar of the mark 2 coupling to allow the mark 3 hook to actually hook under the bar

2. The deep triangular portion of the hook of the mark 2 needs a notch filed or ground into the back of the triangle where it joins the straight bit, allowing this hook to engage under the crossbar of the mark 3 coupling.

I achieved both of these tasks with a Dremel and some small milling tools. Be aware that the crossbar on the mark 2 will be rendered relatively weak because of the thinning operation.
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