Is a re-entry permit valid for train travel to Kuala Lumpur from Thailand?

Yes, a re-entry permit that is already obtained and in your passport is valid for your train journey to Kuala Lumpur via Pedang Besar, as it can be used at all ports of entry. However, if you need to obtain a re-entry permit, it must be done at an airport or through local immigration before departure, as they cannot be purchased at land borders.