Despite both Hong Kong and Mainland China using the same 25 kV AC electrification system for their mainline railways, for many years DF11 diesel locomotives were used to haul the Intercity Through Train that crossed the border.
The reason – the pantographs fitted to China Railways electric locomotives was incompatible with the overhead wires in Hong Kong.
Chinese-language Wikipedia covers the gory technical details.
China Railways SS8 electric locomotives use TSG3 630/25 single-arm pantographs with metal slide plate and straight bows.This is different from the European standards used by Hong Kong’s Kowloon-Canton Railway Corporation, which requires curved bow heads and a carbon slide plate.
The first modified pantograph was installed and tested in March 1999, with the modified SS8 locomotive used to haul through trains from 1999 to 2002 and 2004, but the KCRC believed that even if the TSG3 pantograph was modified, the risk of damaged to the contact network was still high.
In 2000 the German company of STEMMANN-TECHNIK and Datong Electric Locomotive Plant came to an agreement to use the German DSA150, 200, and 250 series pantograph technology. The Chinese produced DSA150 and DSA200 pantographs adopt European standards, with air springs that absorb high-frequency vibration, and are fitted with carbon slide plates.
In 2005 a DSA series pantograph was fitted to a SS8 electric locomotive, and on 3 January 2008 the first China Railways electric locomotive entered Hong Kong for many years – SS8 0191. By 2009 additional services were hauled by SS8 locomotives, and by 2015 all through trains were now electric hauled.
As of December 2012, SS8 electric locomotives that have entered Hong Kong include: SS8-0141, SS8-0148, SS8-0156, SS8-0163, SS8-0166, SS8-0173, SS8-0181, SS8- 0186, SS8-0191, SS8-0192.
Despite the opening of the Express Rail Link in 2018, SS8 electric locomotives still haul Intercity Through Trains into Hong Kong.
The ten SS8 locomotives approved for operation into Hong Kong have been fitted with equipment for the Automatic Warning System (AWS) used on the East Rail Line, but are not fitted with equipment to automatically shut off power through neutral sections.
Instead train drivers have to shut off and restart power through neutral sections according to lineside signage, which is of the same design of that used by the Chinese Ministry of Railways, right down to the simplified Chinese characters.