I was initially on the side of "it's not a rent" (so is unaffected by mortgage state). But thinking about it, I've been forced to change my mind.
My initial thinking was as follows:
The following cards levy fines, but are not considered "rent" for being on the Chance/Community Chest square, and hence set the precedent: card-based fees are not rent.
"Make general repairs on all your property"; "Pay poor tax of $15"; "You have been elected the Chairman of the board - Pay each player $50". Community Chest: "Doctor's fees"; "Pay Hospital fees of $100"; "Pay school fees of $150"; "You are assessed for street repairs".
The following cards direct movement which could land you on an owned position, without giving instructions about rent to be paid, nor what may/should be done if the property is unowned. These cards set the precedent: buying or renting rules are unaffected unless stated.
"Go Back 3 Spaces"; "Advance to St Charles' Place"; "Advance to Illinois Ave"; "Take a trip to Reading Railroad".
My initial point of view was based on these two precedents. They seemed clear.
But then I thought further.
Only one other card directs movement to an ownable square. This card sets the (far weaker) precedent: terms for buying unowned spaces are restated, if the rental terms are also modified on the same card.
"Advance token to the nearest Railroad and pay the owner twice the rental to which he/she is otherwise entitled. If Railroad is unowned, you may buy it from the bank."
The Utilities card remains ambiguous. The possible interpretations that I can see, are:
1) The card's fee is in addition to the rental mechanism; you roll dice to calculate the fee, and also separately calculate the rent, if the property is unmortgaged.
2) The card's fee modifies the rental mechanism; you never pay normal rent, and only pay the fee (as rent) if unmortgaged.
3) The card's fee replaces the rental mechanism; you never pay normal rent, but you always pay the fee, whether mortgaged or not.
Option 1 is the only option you can go for, if you are arguing by the first two precedents, that card fees are not rent; or that rents are normal unless specified.
Essentially, you are arguing that the fee is a fare to get there, rather than a rent to stay there.
And yet, nobody is arguing for this option. Over and above the harshness of getting fined twice for a single event, there is a bigger problem.
When reaching Utilities by this card, if the property is owned and unmortgaged, the rent cannot be calculated as normal, using $4x[the dice rolled to get there], as you arguably did not roll dice to get there. The dice you rolled took you to Chance, and you were sent to the utility by a card! $4x0 would be zero rent. No other card (such as "Go back 3 spaces") can cause you to land on a utility, so it's just this card one which has this issue.
So the card must be stopping the normal rent from happening. Which leaves the other two options. Is it changing that rent to a different amount, or is it getting rid of it entirely and replacing it with a taxi-fare to get there instead?
- the precedent "card fees aren't rent" won't work here;
- the precedent "rent is unaffected unless stated" won't work here;
- the dice reroll seems designed to resolve that rental problem;
- there's no other card which prevents rent payment;
- there's no other property which can still earn when mortgaged;
- the Railroad card seems intended as a parallel;
- the Railroad card doubles normal rent and respects mortgages...
...I'd rule for option 2: that the payment is intended to be a rent, and that mortgages should be respected.