UK SIC 2007 · 42120
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.
Division 42: Civil engineering
Civil engineering is about infrastructure, not buildings. Roads sit under 42110 and railways under 42120. Utility networks for power and telecoms use 42220. The broad code 42990 covers other civil projects, and it is the division's most used UK code. That catch-all holds groundworks and infrastructure contractors. Their work spans drainage, foundations, flood defences and external works. Companies range from national contractors down to small civils firms serving housebuilders. Subcontracting chains run deep here, so much of the register is specialist subcontractors.
Related SIC codes
Who prospects SIC 42120 companies
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. They watch for growth, because growth means a bigger fleet. Recruiters supplying operators and site engineers follow the same signals, as do insurance brokers and surety bond providers. Civils work comes from frameworks and tenders. So a new company often means a management team has spun out to chase one contract. They need bonds, accreditation and plant right away. Utility contractors under 42220 buy in step with fibre and grid investment cycles.
Lead generation for trades & construction →How to read SIC 42120
SIC 42120 is a five-digit code, and the digits are not arbitrary. The first two, 42, are the division, civil engineering. Add the third and you have group 421; the fourth narrows it to a class, and the fifth to the subclass, which is the level Companies House actually asks a company to pick. The whole division sits under section F, construction, the broadest tier there is. Reading a code left to right therefore goes from a whole sector down to one activity.
A company chooses its own codes when it incorporates and can change them on any confirmation statement. It may list up to four, and Companies House does not check whether they are right. That is why 42120 is worth reading as a claim the company made about itself rather than as a verified fact, and why the first-position and any-position counts above differ.
Measured on the August 2026 register, the busiest registered-office post towns for 42120 are London (531), Birmingham (39), Sidcup (39), Romford (37) and Doncaster (35). Those are registered-office locations, which is where the paperwork sits rather than necessarily where the work happens, so treat them as where to start looking rather than as a service area.
Using SIC 42120 as a filter
2,707 companies is 0.05 per cent of the 5,190,464 active companies on the UK register, and ranks 42120 4th most used of the 7 codes we track in division 42. That is the pool before any filter. Whether it is a big market for you depends on what you sell, but it is the number every other filter on this page cuts down from.
2,277 companies put 42120 first and a further 430, 16 per cent, list it in one of their other three SIC slots. That gap is the difference between a tight list and a broad one. First-code-only is the safer filter when you are writing to them; matching any position is worth it when you need the volume.
These companies are spread out. London is the largest single cluster at 20 per cent and the top five post towns together hold only 25 per cent, so the remaining 75 per cent sits across the rest of the country. A regional filter narrows this list without gutting it, which makes it a reasonable code to work territory by territory.
In practice the code on its own is rarely the whole filter. Pair 42120 with an incorporation date to catch companies in their first months, when they are still choosing suppliers, or with a filing deadline to reach them while the year end is on their mind. A worked example for this code: 42120, registered office in London, incorporated in the last ninety days, which turns a 2,707 company pool into a list you can write to in an afternoon. Add the poor-website filter if what you sell is a website, or a recent change of officer if you sell to whoever just took the job. Every company you pull carries the Companies House record, the director's name, a verified website and a verified email, and the ones we cannot complete are not charged.
Where SIC 42120 clusters in the UK
ACTIVE COMPANIES BY REGISTERED-OFFICE POST TOWN · COMPANIES HOUSE REGISTER, AUGUST 2026
SIC 42120 companies, as leads
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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.
Can one company hold codes across construction, civil engineering and plant hire?
Yes. Companies House allows up to four SIC codes and does not restrict which divisions they come from. A contractor doing building work, civils and equipment hire can list codes from divisions 41, 42, 43 and 77 at the same time. You set codes at incorporation on the IN01, and change them on the CS01 confirmation statement. There is no approval step. Updates take effect once the statement is filed.
Find UK SIC 42120 companies
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