Railings is that gorgeous off-black with a soft blue-grey undertone — not a flat, dead black, which is exactly what makes it such a popular choice for front doors, railings (the clue's in the name) and moody joinery. If you want to get close in Dulux, you've got two solid options.
Dulux Basically Black is your best bet at ΔE 1.9 from the original. Anything under 2.5 is what we'd call very close, so at 1.9 you're into territory where the difference is genuinely hard to spot once it's on the wall and dry. Its LRV sits at 7.3, which keeps it firmly in deep, light-swallowing territory — much like Railings itself.
If you want a fraction more grey in the mix, Dulux Charcoal Drift lands at ΔE 2.2 with an LRV of 7.6. Still very close, just a hair softer and slightly lighter than Basically Black. Between the two, I'd steer you to Basically Black for the tighter match.
Now the honest caveat: a colour match gets you the hue, but it won't replicate Railings' *finish*. Farrow & Ball's depth comes partly from their pigment load and the way their paint sits — particularly in Estate Eggshell or Exterior Eggshell on woodwork. Dulux in their standard ranges will read a touch flatter. For exterior doors and railings especially, that finish difference shows more than a 1.9 ΔE ever will.
My advice: if you're doing a big wall area and want to save money, Basically Black is a cracking stand-in and nobody will know. But if it's a hero front door or feature joinery where the finish is on show, it's worth paying for the real Railings in the right eggshell. Always test a sample on the actual surface in your own light before committing — dark colours shift more than you'd think between morning and dusk.