https://reopen.europa.eu/en/map/BEL/7001Как ни странно, но вот здесь не врут:
All travellers from a "red" zone, who are not resident in Belgium, are subject to the requirement for a
pre-departure negative PCR test.
The test must be taken within
72 hours before departure.
Children aged 11 and under are exempted.
This test is also mandatory for passengers connecting via Belgium to a Schengen destination (including Switzerland, Norway, and Iceland).
Вся Европа сейчас в красной зоне, так что запросто на поезд тест нужен. Это - теория. Практически же: не знаю никого, кто бы на поезде влетал на проверку теста