The ICC was specific as to Common Carrier Steam Railroads. The Interurbans and Electrics that you mentioned were, even at that time, making more money generating electricity and selling it than running a railroad.
Personally, I think that the definition gelled during the USRA era when the Government paid reparation to the Steam carriers after the USRA's demise but the Electrics and Interurbans were specifically excluded from these reparation payments. The Steam carriers went to extended gyrations to be included as common carriers and to get their money, even those who were under the USRA for less than a year, but connected with Trunk Line carriers who were under the USRA for the duration. Even though the ICC was at first hesitant to pay those like the Swamp Gas and Southern Railway, these smaller lines took the ICC to the Supreme Court and the Lawyers won the day, and everybody got some cash, except the Electric, Interurban and Street Railways.
The definition of Common Carrier came to mean a Transportation company that was compelled to haul any freight that you wish to ship, in any amount, as long as you are willing to pay the charges. However, the wording of the Valuation act specifically excluded the Electric, Interurban and Street Railways and those Railways that I described in my earlier post.
What was the advantage of being deemed a Common Carrier? Easy, you could insist on a split of the line-haul fees rather than just having to charge a switching fee. In other words, MORE MONEY.
Rick