Here is some comparison pictures, Dapol, Mainline, Replica and for effect, Hornby. It's my opinion they are some differences... Side elevations (top to bottom is Dapol, Replica,Hornby and Mainline ). Ignoring paint finish (even though the maroons are very similar) First the frames.. Mainline (with its Airfix clipfit) is the same as used by Dapol.. Here's the ck for both mainline and Dapol... The giveaway is under the Dapol chassis... Now the Replica one... This is a partial step up in quality, separate roof vents and separately fitted handrails on the roof (a bit ahead of its time)..but non flush glazing. The body length is longer, indeed its .5mm short of the Hornby one, but 1mm longer over buffers. Note the Replica one, It's stamped Bachmann under neath... (top to bottom is Dapol, Replica, Hornby, Mainline). The ends are distinctly different between Dapol/Airfix and the Replica (Bachmann) & Hornby ones.., note the Replica one has separately fitting handrails on the roof.. To the bogies.. the detailing, mould lines, coupling fitting, fixing pins are the same on the Mainline /Dapol ones, Replicas bogie looks the same as that used under Bachmanns Stanier full brake. Finally here is the Mainline Full brake, next to the Replica SO and the New Porthole... the Porthole has all new bogies, the other two look the same tooling. Interesting although identical the Replica bogie has a "2" on the tooling... If I were to guess the minds of the early 1980's designers... with Kader already having a Stanier full brake and having acquired a BSK and CK from Airfix, adding a Stanier SO/FO (and who knows maybe a 12 wheel dining car ???) wouldn't that complete the rake ? The 12 wheel dining car came out under Dapol but looks distinctly Airfix to me. So what would make the Replica SO a "second".. only in that to me it looks the odd one out, and the Stanier bogie having a "2" on it. With Dapol churning out the BSK/CK and Replica with a batch of odd ones... why would Bachmann retool a new rake, even with an SO in hand ? 30 years later it's Hornby that have made the full rake, ironically with the dining car that looks Airfix.
As the maroons across the ranges are similar, I mix and match without any issue, though the Mainline one I tend to put in a mis-matched rake of other styles as the yellow lining isnt the best.
hope that helps.