What is the answer to the 4th riddle on the train?

It's 565

Riddle to enter 2nd train carriage: Black’s 100, blue’s 10, red’s 5. This lady is all in. If to pass this door you strive, Find the total the dame’s holdin’."

Solution: Notice that “all in” and “holdin” suggests betting in poker. The note that was in The Conductor’s last message from compartment 103 had a poker chip next to it. The note read: "Eleanor Neale: The Murder in the Graveyard.” Find this JJ-sponsored video on youtube and watch the beginning. Note that behind Ms. Neale on the desk, is an infinity rose, like one of the prizes available in this contest, and 3 stacks of poker chips: 5 black, 7 red, and 3 blue. Do math: 5x100 + 7x5 +3x10.

Answer: 565