Prior to the first part of the high speed line opening, Eurostars could go 6 different ways to the tunnel:
The most commonly used route was Waterloo - Bromley South - Orpington - Sevenoaks - Tonbridge - Ashford - Cheriton. Trains would however occasionally be diverted and run Waterloo - Bromley South - Swanley - Maidstone East - Ashford - Cheriton, and even more occasionally, but still used from time to time was Waterloo - Bromley South - Swanley - Bat and Ball - Sevenoaks - Tonbrisge - Ashford - Cheriton.
There were 2 variations of each of these routes, according to whether the trains went from Waterloo - Bromley South via Herne Hill or via Catford.
All of these routes were extensively upgraded prior to the introduction of international services in 1994. Power supplies were upgraded, continously welded rail installed, and in many locations line speeds were upped to 90 or 100mph, this being as much as the infrastructure would allow. The loops at Bickley Junction were multiplied to quadrupled track too at that time.
Power demands were high due to the extremely tight pathing constraints in the suburbs often required eurostars to run 2 trains in the same direction in a section, with Bruxelles services usually running 3 mins behind the Paris ones.
Also, class 92s used on cross channel freight used a hell of a lot of current, so much so that they are still barred from going via Herne Hill.
At the same time, Redhill - Tonbridge was electrified and upgraded, and most international freight in the daytime still goes that way.
The original plan was to subsequently construct a high speed link though Kent and South East London, with extensive tunnelling emerging in the Peckham area, then running over 'classic lines' into Waterloo. This route however ran through the back gardens of a lot voters in a lot of marginal tory seats and soon lost out to an alternative through marshland in the Thames estuary.
While we in Britain all argued and took 13 years to build ours, the French had their high speed line to the tunnel ready for Eurostar's introduction in 1994, and the Belgians finished theirs a couple of years later. Some would say we missed the boat somewhat there, or maybe the train that replaced it...