From Railway's documentation, they say that they support aliases:

"At this time, Railway supports CNAME Flattening and ALIAS records." here

Brody has said in several threads that squarespace does not support CNAME flattening at the root here, although this is true, there is a way to get a squarespace domain to work with railway directly using an alias for the root. You need two records:

Record 1:
Host: @
Type: ALIAS
Priority: --
Data: ______.up.railway.app

Record 2:
Host: WWW
Type: CNAME
Priority: --
Data: ______.up.railway.app

In your railway -> your project -> your service -> settings -> networking -> public networking setup your domainHere.com as record 1, and add another custom domain to as www.domainHere.com. The Data for record 1 and 2 will be different! Setup record 1 as @ and record 2 as www.

Takes just a few minutes. You should immediately see, "Issuing TLS certificate" and then "Waiting on DNS" this will go on for a few minutes till it says "Setup Complete"

Hope this is helpful!