There was an exact same question but no public follow-up: Referencing Service Variables in Shared Variables - Railway Help Station

I ran into the same problem. To elaborate, say:
I have a Redis service deployed, and I will have multiple services using it. It is very intuitive to make the Redis.REDIS_URL a shared variable, so that I don't have to manually set this for all services using the Redis.

But now, say:
We set in shared variable so that:
shared.REDIS_URL = ${{Redis.REDIS_URL}}
We have a service Backend. And we add the env var to it:
REDIS_URL = ${{shared.REDIS_URL}}
The actual REDIS_URL in Backend will be empty.

I guess the work to enable this feature is non-trivial, due to possible circular referencing. But it is not impossible to tackle, and I believe it can be quite a cool feature that saves time.