If you are referring to a Japanese Government Disability ID, you cannot get one without Residence status in Japan (visiting on holiday as a foreigner does not qualify for this status). While this means you don’t have access to discounts related to disabilities, you are not required to have the ID to get assistance from the Train Attendants if you need it (e.g. such as slope assistance for wheelchair users).

You may find additional useful information in the recently updated section “Wheelchair Accessible Trains and Subways in Japan” from the Accessible Japan website here: https://www.accessible-japan.com/wheelchair-accessible-trains-and-subways-in-japan/