Bug MC-275365 allows us to see which minecarts the game considers to have a passenger.
Turns out all minecarts with the passenger slot taken up by a block are treated as empty.
This includes

• Hopper Minecart
• Chest Minecart
• Furnace Minecart
• Command Block Minecart
• Spawner Minecart
• TNT Minecart

This doesn't make sense, since these minecarts do have a passenger (the block inside them) so the games internal logic should treat them the same as regular minecarts with passengers.
I don't really know what effects the game applies to minecarts that "have a passenger" besides the speed boost, but it makes sense for all these minecarts to be detected as such.

Reproduction Steps

• Set minecartMaxSpeed to 1000
• Build a looped track
• Place any of the mentioned minecarts
• Check their speed with the /data get entity command
• Give them a passenger with /ride command
• Check their new top speed

Observed Results

• You can see that if you force them to have a passenger with the /ride command, their top speed increases

Expected Results

• These minecarts would already be treated as full, and posses all the behaviors of occupied minecarts