Ghost Train - Designing

What impresses me about games like Ridiculous Fishing and even Angry Birds is creating a fun, engaging experience with a few simple mechanics.  I want to keep the design for Ghost Train (this is just the working name, btw) simple but fun.

Here are the elements that I want to include:

• A moving train.  Trains move from city to city on the map.  A train might have 3-7 passenger cars.
• Ghosts.  A ghost is on the train and is sucking the souls out of passengers.  There are different types of ghosts, each with unique abilities.   
• Ghost Hunters.  "Line Bulls" as John Harper describes them in Ghost Lines.  Players will have 2-5 of these hunters, each with their own specialty.
• Hiring.  Hunters are hired (maybe from a bar or employment line).
• Different train lines which get progressively more difficult.
• Upgrade system/equipment buying.

The action part of the game, I envision as a relatively fast paced fight between the Hunters and the Ghosts.  This is kind of a twitch action, but I also want it to feel somewhat puzzle like.  This is something between a boss fight in World of Warcraft mixed with a rhythm game.  You are constantly selecting characters from your crew, positioning them and activating their power, then moving to the other.  You need all characters working in concert to capture the ghosts.

Narratively, I imagine the battle happening something like this:  The characters enter the train car.  It's full of passengers, but no ghost to be seen.  The librarian moves to each part of the car, putting on his goggles and scoping out looking for Aetherial matter.  He spots a ghost sucking the soul of an unwitting passenger.  The player moves the librarian out of harms way quickly and moves his damage guys in.  They start hammering away at the ghost with spirit hammers.  The ghost isn't just going to stand there and take that so he attacks the nearest hunter and starts draining his spirit.  The hunter's getting weaker and weaker, so the tank moves into position, attracting the attention (and wrath) of the ghost, leaving the damage guys alone.  The ghost is getting weaker and weaker.  Keep going and the thing will evaporate from existence...but you need to capture it to get the reward.  So, when it's weak, but not dead, you bring in your trapper, moving him into place and activating the spirit trap.  After a couple of tries, it sucks the ghost in just in time.  All the while the passengers keep sipping their tea unaware of the danger they were just in.