According to network operator Infrabel, a Eurostar train bound for London tore down part of the cable support just outside the Belgian capital, blocking the high-speed line.
A Thalys service to Paris behind the Eurostar was also halted, and 300 passengers had to be transferred to another train to return to Brussels and await a replacement.
The 600 passengers on the Eurostar were given water and eventually taken back to Brussels, the Belga news agency said.
Thalys reports long delays on its Paris services.
? Due to the power failure on the Belgian line, we're advising people not to travel this route today. In addition, these trains have been cancelled:
?#ES9132 12:58 LDN>BXS
?#ES9141 14:56 BXS > LDN
You can change your ticket for free or refund here https://t.co/1g1leNzM9s
— Eurostar (@Eurostar) 24 July 2019
Eurostar says that many services between London, Lille and Brussels were cancelled on Wednesday.
On Thursday the services were still listed as being severely disrupted from Brussels.
Anyone who has a ticket for those trains can either exchange it for a later date or ask for a refund.
Suite à un incident avec une rame Eurostar à Halle (Belgique), le caténaire de la ligne à grande vitesse est touché.
Il y a une forte perturbation de trafic.
— Thalys (@thalys_fr) July 24, 2019