What are the differences between the game modes?

The below findings are based on personal gameplay of the Beta 15 version on Steam.

In Normal (previously Commuter) Mode:

The scoring timer in the top-right will measure total passengers served
You recieve a budget increase (i.e. an upgrade) at the end of every Sunday, irrespective of how many passengers were served
If a station becomes overcrowded, you lose the game
The aim is to keep your system running for as long as possible before the inevitable overcrowding occurs

The scoring timer in the top-right will measure passengers served per day (it has a timer, so you need to maintain the rate for a whole day)
You receive a budget increase when you achieve efficiency milestones, i.e. you are maintaining a certain number of passengers served per day
If a station becomes overcrowded you do not lose the game - this means you can play indefinitely
The aim is to maximise efficiency, however, there is no loss condition so this is effectively a sandbox design mode

Scoring and budget increases are as per Commuter Mode
Line placements are permanent; you can extend a line from its end, but you can't "un-hook" from an existing station or delete the line. This means that any closed-loop track is completely permanent.
Line upgrades are also permanent; if you attach an additional train or carriage to a line, you can't then move it to another line (cReddit where credit's due)
Before a new station appears you'll see a faded 'ghost' of where it's going to pop up
The aim is the same as commuter mode, but with the added challenge of being unable to rethink your design once it's laid