Norwega,
I too have been researching monorail and here's what I know so far:
1-Yes, you can put the transformer over or on top of the cabinet. Where is the electrical connection coming from?

2- I think you are allowed to put the transformer anywhere it is accessible, so on top behind the molding should be okay. The electronic transformers are really small and easy to hide.

3- The lamps are plenty bright, IMO. Most of the low voltage monorail I've seen on display has 50 watt fixtures which put out a good amount of light. For my little peninsula I'm going with 20 or 35 watts down-lighting pendants.

Think about how you are going to get power to the rail. My rail is going to run very close to the top of a cabinet, so I'm just going to have the low voltage wires run from there to the end of the rail. They'll be visible, but not too long. I have an open-beamed ceiling so a little bit of an industrial look will fit right in.

The transformer needs to be accessible. If the molding above the cabinet goes all the way to the ceiling and completely encloses the space there such that you'd have to remove the molding to get at the transformer, that won't fly.

Voltage doesn't have much to do with the amount of light produced; wattage is what you need to pay attention to.

"... spots looking towards the work areas of the kitchen."

I wouldn't expect a monorail over the island to provide task lighting for perimeter counters. Anyone between the island and the counter would be right in the path of the light, working in their own shadows when facing outward and getting hit in the eye with glare when facing the island.

I recaculated the ceiling and molding height, and I'll only have an inch between the top of the fridge cabinet molding and the ceiling, so I guess the transformer can't go there.
I was going to have the power installed prior to the drywall going in on the ceiling.
Where would you recommend the transformer be located?
It looks like 8' of rail would work over the island, in a C shape, using pendants for the eating area, and spots towards the work area of the island.
Has anyone worked with "Brilliant Lighting.com"?
{{!gwi}}
{{!gwi}}
{{!gwi}}

You could place the transformer inside an upper cabinet that doesn't get used much; the one over the fridge would be a candidate. If you'll have ductwork going through the cabinet over the stove, that might also be a good spot. An unfinished attic above would work, if there is one. The basement could work too, so long as voltage drop is accounted for.

You could have a junction box installed inside one of the cabinets and mount the transformer with it. Don't forget you have to wire a switch loop to that box.

I would use one of the cabinets closest to either end of the rail. The fridge looks too far away and you'll have feet of low voltage cable running from there.