Construction of railways and underground railways (SIC code 42120)
Construction of railways and underground railways is UK SIC code 42120. It sits in division 42 (Civil engineering) of the SIC 2007 list. On the August 2026 Companies House register, 2,707 active UK companies list 42120 among their SIC codes. 2,277 of them list it first.
2,707active UK companies list SIC 42120 among their codes on the public recordCompanies House · counts as of August 2026
SIC 42120 in numbers
2,707 companies is 0.05 per cent of the 5,190,464 active companies on the UK register. 42120 is the 4th most used of the 7 codes we track in division 42.
2,277 put 42120 first and 430, 16 per cent, list it in one of their other three slots. First code only gives the tighter list; any position gives more of them.
ACTIVE COMPANIES BY REGISTERED-OFFICE POST TOWN (WHERE THE PAPERWORK SITS, NOT NECESSARILY THE WORK) · COMPANIES HOUSE PUBLIC RECORD, AUGUST 2026
London is the largest cluster at 20 per cent and the top five post towns hold 25 per cent, so the other 75 per cent sit across the rest of the country. A regional filter narrows the list without gutting it.
A worked example: 42120, registered office in London, incorporated in the last ninety days, from a pool of 2,707. Each lead you get arrives with a working website and a real email, and the director's name when the record has one. If we cannot complete one, you pay nothing.
SIC 42120 companies, as leads
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Civil engineering is about infrastructure, not buildings. Roads sit under 42110 and railways under 42120.
Related SIC codes
How to read SIC 42120
SIC 42120 reads as division 42 (civil engineering), group 421, class 4212, then the full five digits, inside section F, construction. A company picks its own codes, up to four, and Companies House does not check them, so 42120 is the company's own description of what it does, and 2,277 list it first where 2,707 list it anywhere.
Who sells into division 42
Plant is at the heart of selling to civils firms. Hire companies, excavator and attachment dealers, fuel suppliers and operator training providers all work this division hard.
About SIC 42120
How many UK companies use SIC code 42120?
2,707 active UK companies list 42120 (Construction of railways and underground railways) among their registered SIC codes. Each company may register up to four. Of those, 2,277 put it first. The figures come from the August 2026 Companies House register snapshot, and count active companies only.
Is groundworks a division 42 or division 43 activity?
Both codes exist and the split is genuinely fuzzy. Site preparation and excavation is 43120. Broader civil engineering work fits 42990. Many groundworks contractors choose 42990, because their scope covers drainage, roads and structures. Others use 43120 for enabling works. Listing both is common and allowed within the four-code limit. Nothing is enforced on the choice. It is a self-declared classification.
Put SIC 42120 to work
Find UK SIC 42120 companies
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