Do I get a refund for parts that land on rails?

No, you don't. "On rail" parts which go too deep into the atmosphere just disappear. This even happens when they would have survived the landing (for example because they have parachutes attached). To recover a part you must supervise its landing. This means that it is currently hard to recover the first stage of a rocket, because you can not supervise both the upper stage on its way into orbit and the lower stage on its way back to ground. So no Grasshopper launch system yet.

However, I did construct a launch system which has a first stage of solid rocket boosters (which are dirt cheap) and a reusable second stage which can bring a payload of up to 15 tons onto a sub-orbital trajectory. After leaving the atmosphere I can detach the payload, use the payloads engine to get it into a parking orbit, and still have enough time to switch back to the second stage before it despawns so I can land and recover it.

Here is a screenshot of the system with a 15 ton payload on top.

Also, you can only recover a part which has a command pod attached to it. So if you want to recover a stage, you need to add at least a probe core to it.

I just got told that there already is a mod for that - DebRefund:

DebRefund refunds you for dropped stages and such that have enough parachutes to land.

Basically, put parachutes on stuff!

Funds will get added, and messages will appear in the message box, as debris is "recovered". There are 3 regions of recovery, based on mass to parachute drag ratio.

Green is landed safe, and does 88% to 9% refund based on distance from KSC, yellow is landed with some damage, and yields 90% to 50% of the green level, and red is it slammed into the ground, no refund.

Green is <6m/s touchdown speed, Yellow is <10m/s touchdown speed.

StageRecovery does pretty much the same thing:

What can this mod do:

• Recover funds from stages that have enough parachutes to drop the terminal velocity to a safe level
• Two funds recovery models: Flat Rate, where once the velocity drops below a cutoff a flat rate is returned; or Variable Rate (default), where the velocity within an intermediate range determines the recovery percentage (following a quadratic relationship)
• Works with Stock parachutes and Real Chutes parachutes
• Recovery percentage (flat rate model) is configurable (default of 75% of the normal for the current distance from KSC, 100% if a probe core or Kerballed command pod is attached)
• All cutoff velocities are easily configurable with in-game sliders in the in-game settings menu
• Recovery does not require that parachutes are deployed, only attached (but it is a good idea to deploy them at low altitudes)
• Recover Kerbals in dropped/returned stages
• Recover science in dropped/returned stages
• Integrates with stock message system (reports can be disabled in the setting file)
• Easy to use, non-intrusive GUI for viewing Recovered and Destroyed Stages.
• Editor helper to help you figure out just how many parachutes you need.
• Has an easy to use API that other mods can tie into to be notified of recovery events (success or failure)
• Support for KSP-AVC for version checking
• Powered recovery of stages. Land with rockets, instead of just parachutes!