sql server 2012 – how to display running count of records in query

I am working on SQL SERVER 2012, and I have been trying to figure out how to include a running count of records in a group by statement. For example, the query would look something like:

SELECT CUSTOMER_ID, TRANSACTION_ID, COUNT
FROM TRANSACTIONS
GROUP BY CUSTOMER_ID, TRANSACTION_ID
ORDER BY CUSTOMER_ID, TRANSACTION_ID

And the output would look something like:

10 100 1

10 108 2

10 125 3

10 140 4

12 102 1

12 103 2

12 138 3

15 135 1

I appreciate any advice on how to query this information.