Train experts: What is going on in this situation?

They may have been replacing the entire panel of track over that crossing. But that takes more like an hour.

Crossing.

The train gates were down and flashing red.

OBVIOUSLY you were required to stop.

Many things can go on train tracks other than just trains.

Yellow truck on the tracks

Colour of truck means little.

with a hook or something

Obviously truck has a something or a hook as part of it. From your brief description a crane.

What was happening there? Excellent question

Probably needed for something along the tracks. HINT a crane is used to lift something.

Why would there be a delay this long?

They could be in position for days at a major incident.

Could be something like a tow truck. Similar to what would be used to haul a bus or big rig truck.

https://www.youtube.com/watch?v=zny8w0nmA0g

Truck waits just like trains for permission to proceed down tracks when safe to do so.

could be just one of many specialized things going to or returning from an incident.

https://www.youtube.com/watch?v=tlI1XuqYWi8

Does take a few minutes if the vehicle has the ability to convert to road method and get off or onto the tracks.

https://www.youtube.com/watch?v=Wnt6n0lJYOk

The railroad owns the tracks and has NO obligation to allow traffic to go over them. So if they want to close the tracks to do some work, it is up the the individual drivers the find another route if they are in a hurry!

Looks like a maintenance vehicle. It lines up rails or ties that are out of alignment.

Where was the crossing so somebody here can investigate?

A yellow truck with a hook is commonly referred to as a crane. I would imagine it was lifting something

Filming

Presumably there was damage to the track or the overhead power cables; and the truck was repairing the damage.

Fifteen minutes isn't especially long to wait; would you have preferred they didn't fix it and people were killed?