Refactor & Aggregate specs for merge_from_train_Ref_service

Summary

There's a bunch of duplicated specs for merge_train_services. Let's use some shared examples for collect them

!125921 (comment 1464523506)

Proposal

Additional details

Some relevant technical details, if applicable, such as:

• Does this need a feature flag?
• Does there need to be an associated instrumentation issue created related to this work?
• Is there an example response showing the data structure that should be returned (new endpoints only)?
• What permissions should be used?
• Is this EE or CE?
  • EE
  • CE
• Additional comments:

Implementation Table

Group	Issue Link
backend	👈 You are here
frontend	#123123

Links/References