Twenty years after the station came into use, expansion and remodelling of the approaches was already on the cards. The SER decided to widen the railway bridge to carry a total of ten parallel tracks, twice the original number. Two of these tracks were to be sidings, and the original Saxby & Farmer signal box was to be dismantled and replaced during the works by a much larger affair. To cope with intensive locomotive movements at the terminus, engine facilities were to be improved by the addition of a second, smaller turntable on the south side of the river, which was positioned on the opposite side of the approach tracks to the motive power depot. This cramped site not only accommodated the second turntable, but was also host to a single engine stabling siding, and Cannon Street No. 2 signal box. With reference to the latter, this was of SER design, with sash-style windows and a timber upper half, and replaced an earlier signal cabin placed within the apex of Stoney Street Junction. The lack of space necessitated the cabin to be partially suspended upon an iron-strut framework over the turntable. The works also included new signal cabins at Borough Market and Metropolitan Junctions, these being of SER and Saxby & Farmer designs respectively. During these alterations, a fourth track was also installed on the eastern side of the triangle. Completion of bridge alterations came on 1st February 1892, but re-signalling of the layout was not complete until April of the following year, the enlarged Cannon Street signal cabins, Nos. 1 and 2, being commissioned on 22nd of that month. With reference to Cannon Street No. 1 Box, this was also of SER design, with sash-style windows set into timber-clad sides. It was an elongated version of those typical SER cabins erected at intermediate sites, stretching some 110½-feet over all ten lines entering the station. As per the cabin’s predecessor, it was suspended upon a lattice framework, affording the signalmen operating its 244 levers a clear view of all engine and train movements. Finally, as a result of these works, the station had acquired an additional two platform faces. Formerly, a cab road ran the length of the platforms (as was the situation at Charing Cross), sandwiched in-between the original Nos. 3 and 4. However, the extent of this was halved to allow the insertion of an additional single-track bay. Cannon Street now had ten platform faces, but peculiarly, numbering remained 1 to 9, west to east – the two faces of the then recent single-track were treated as a single platform (No. 4). Consequently, the whole surface which encompassed platforms 3, 4, and 5, was extended southwards to meet the 1893-commissioned No. 1 signal box. Platforms lengthened onto the railway bridge had extensions of timber construction.

Under SE&CR auspices, a bridge strengthening exercise was initiated in 1908, but during this company’s ownership, there was a period of decline. World War I saw draconian economical measures imposed on services. Continental trains were withdrawn from both Charing Cross and Cannon Street in November 1914, and such workings lasted at Victoria until April 1916. The revised, skeleton-like New Years Day 1917 timetable also saw the near total abandonment of Charing Cross services running via Cannon Street. Furthermore, Cannon Street was now closed on Sundays, from 15:00 on Saturdays, and in-between the morning and evening peak times during the week. During closure, the station effectively became a motive power depot, for it was used for exchanging and stabling locomotives and their crews. The engines in question were hauling trains of war supplies bound for Dover.

The Southern Railway brought substantial changes to those lines connecting the ex-SER London termini. During August 1925, the track layout between Cannon Street West Junction and Charing Cross inclusive was extensively revised for impending electric operation. Although World War I had put paid to the practice of running all Charing Cross services via Cannon Street, when the SR took over the network, the track layout was still geared for this. Third rail was installed at both ex-SER termini for the planned commencement of electric services on 1st December 1925, but Cannon Street retained its existing SE&CR track arrangement. This was a temporary measure: the plan was to completely rebuild and re-signal the Cannon Street approaches and platforms in the summer of the following year. In the interim, the steam-era layout could still be converted – if only partially – to electric operation, platforms 5 to 8 subsequently receiving third rail. Scheduled electric services from the two mentioned termini to the Hayes, Bromley North, and Addiscombe branches, and Beckenham Junction and Orpington, in fact started later than planned, on 28th February 1926. A lacking electricity supply was blamed for the delay. Cannon Street continued to be closed on Sundays and on Saturday afternoons. It was decided that, after the Saturday closure on 5th June 1926, work could commence on the substantial infrastructure re-arrangements at the terminus, for a period of over three weeks, in connection with electrification. The SER-designed No. 1 signal box of 1893, which straddled the approach tracks on the railway bridge, was dismantled, and No. 2 box south of the Thames became redundant; the latter’s demise was the result of the abolition of Stoney Street Junction. The SR electrification track plan had a heavy emphasis on non-conflicting movements, which was to be afforded by a series of parallel train movements. Thus, the northern end of the triangular layout was no longer a junction, merely a divergence of tracks. The SE&CR had, with great success, initiated a system of parallel working back in February 1922, to tackle conflicting train movements during the peak time congestion. Shortly before the works had begun, Cannon Street engine shed had been subject to closure; the abandonment of reversing Charing Cross services at the City terminus, combined with the conversion of suburban services from steam to electric, made this cramped depot obsolete. It was subsequently flattened, but the 1892-installed locomotive turntable west of the approach tracks was retained, and engines were able to access the nearby Ewer Street Depot. The site of the engine shed became home to one of the SR’s steel-framed, red-brick sub-stations, which comprised three rotary converters. Each rotary converter was rated at 1,500 kW, and converted current delivered from Lewisham substation (which was ultimately fed by Deptford Power House) to 660 Volts D.C.

Of the platforms, rebuilt in the then prevailing concrete, nine became eight in the re-arranged layout. The latterly-conceived platform 4, which comprised a single-track terminating half way up the trainshed, in addition to a pair of platform faces, was removed. Platform numbering was reversed to 1 to 8 from east to west, and platform Nos. 1 to 5 received third rail for electric suburban services. The largest platforms, Nos. 6, 7, and 8, were allocated for main line steam workings, the longest surface measuring some 752 feet. Of the newly re-laid track, much of it had been pre-assembled at New Cross Gate and transported to the terminus in bulk loads. The same practice was later employed during the re-arranging of the Chislehurst Junction lines in 1958. The layout was to be controlled by a 45 foot-long power-operated 140 miniature lever frame housed within a two-storey high pitched-roof signal cabin. This was built on the western side of the railway bridge, just in front of the imposing west tower, and was of all-timber construction to keep weight to a minimum. The rebuilt layout came into use for services at 04:00 on Monday 28th June 1926, four-aspect colour lights now being in use.