DHS doesn't only operate TSA and CBP. At many federal buildings, DHS accepts trusted traveler cards as ID (check beforehand to make sure, as each agency and facility can have exceptions) and most other federally restricted areas where DHS operates security. A notable exception is CBP at airports to enter the US, where you need a passport.

Trusted traveler cards seem to be acceptable for I-9 purposes, as "list B" documents. The interpretation seems to be they can verify identity, but not ability to work. This seems ironic to me since it states nationality right on the front of the card, but it seems as foreign nationals can get trusted traveler cards without work authorization, these cards aren't eligible for "list A" status

While it might come as a surprise, even to many who take trains regularly, but TSA also regulates security for certain train travel (eg. on Amtrak) and requires photo ID that should be shown on request to TSA or the rail operator.

Trusted traveler cards issued by DHS should qualify, although given that TSA doesn't sometimes even recognize these cards at the airport I wouldn't necessarily count on them recognizing these cards on a train.

Trusted traveler cards are not on the list of acceptable ID when applying for passport or passport card. However, when required ID documents are not available certain kinds and combinations of secondary documents may be sufficient. While the State Department doesn't enumerate all acceptable secondary documents, I would assume a valid, unexpired trusted traveler card would be considered strong secondary ID. But I wouldn't expect State to automatically accept it.

State rules vary. Michigan lists trusted traveler cards as acceptable ID to present at the DMV. Typically trusted traveler cards don't make the list of acceptable identification at most state DMV's to apply for drivers' license or state identification. However, as with passport applications, some states allow secondary verification of ID when primary documents are not available, and trusted traveler cards may work in some cases.

This depends strongly on state laws and the person checking the ID. Some states (like Massachusetts, as posted above) specify what forms of ID have special legal status and don't include trusted traveler cards. Even in states where this is not the case, you're dependent on the ID checker knowing what on earth is a trusted traveler card.

For this use, it would have been more useful if trusted traveler cards were more clearly marked as issued by DHS, or even the US Government. However, anyone familiar with checking ID would recognize the numerous security features on these cards.

One can argue about whether stores can or should ask for ID for credit card or check payments, but this depends entirely on the store/clerk. Based on my history of showing passport card instead of driver's license, most places are OK, with the occasional clerk that thinks the only kinds of ID that exist are driver's licenses.

Also, passport cards/trusted traveler cards do not show the carrier's signature. So any place that wishes to do signature verification will force you to use drivers' license or passport.

States with voter ID laws also vary. In Virginia, any federal government issued photo ID is acceptable; although I would strongly suggest having a backup in case they don't know what on earth is your trusted traveler card. Other states like North Carolina do not have trusted traveler cards on their list of acceptable photo ID for voting.

To take ETS administered tests like the GRE, photo ID bearing signature is required. This eliminates passport cards and trusted traveler cards.