currently you can only add 1 instance of an addon, like redis, and it will set the environment variables like REDISURL, REDISPORT etc.

in heroku for example, you can add 1 instance of an addon and it work the same way railway does now, but you can add more instances of the addon and it will set the environment variables like

instance 1:
REDISURLBLUE
REDISPORTBLUE
etc

instance 2:
REDISURLRED
REDISPORTRED
etc