In message ,
Tom Anderson writes

Why were there two Holborn Viaduct stations, then? AIUI, the low level
station was the first to be built,

The high level terminus was opened on 2 March 1874; the low level
through station (Snow Hill) was opened 1 August 1874. Basically they
were planned as a complementary pair of stations.

so why did someone see the need for another station in more or less the
same place?

Essentially, the High Level was intended as a terminus for LCDR mainline
services, including various boat trains to the continent, while the Low
Level was primarily for suburban services, many terminating at Moorgate.
The High Level was actually very small - just 4 platforms designed for
half-length trains. The other half of each train was a West End portion
(for Victoria) with the portions being split or combined at Herne Hill.

Was it just to provide more capacity?

That too. In order to finance the line the LCDR sold running powers to
the GNR, Midland and the LSWR, so there was an enormous range of
services on the line - trains from Kingston/Richmond/Wimbledon
(terminating at Ludgate Hill), Herne Hill to King's Cross and Barnet,
GNR services from Hatfield to Ludgate Hill, Muswell Hill to Woolwich,
Midland services between Hendon and Victoria via Ludgate Hill, etc.

I can imagine that reversing lots of LCDR trains at the low-level
station would make it rather hard to run a high-frequency through
service as well.

I don't think they ever reversed at the Low Level station - trains on
the Snow Hill line would either continue north via Farringdon or would
terminate at Moorgate. Congestion was so bad at the latter that it was
often quicker to walk from Snow Hill in the late 19th century.

I can't imagine a London where there was the space to go round building
stations willy-nilly like that!

I don't think it was the LCDR's wish to end up with three tiny terminals
in close proximity (Ludgate Hill, Holborn Viaduct and St Paul's - the
last of these later being renamed Blackfriars). Basically, they had been
bankrupted by the cost of their City extension and with property prices
so high in the area, all they could manage was to build very small and
mean stations whenever an opportunity arose.
--
Paul Terry