sensor suitability for train project

We have to install solar panels on the train rooftop and they have also asked for speed measurement of the train but,we can have no arrangement along the wheels or engine to know speed of the train. Can anyone suggest me what sensors I can use to know speed of the train having access only to rooftop.
Can I use radar like arrangement where the radar will be pointing downward (may be along the rails…).
May be accelerometers as well but, they are not accurate as far as I know…(may be I am wrong);
I have these sensors right now:
http://www.ebay.in/itm/252521417437?aff_source=Sok-Goog
Anyone could suggest me how I could get started with coding.

I would first consider a GPS module with the added advantage of determining location.

What are you going to do with the RF modules?

I agree, GPS will be best for speed (although not if the train is in tunnels!). Are you going to use the RF modules to transfer your measurements to another computer somewhere in the train? If there is no existing Arduino library for those modules, you will spend a lot of time designing a radio protocol. Better to use something that is more expensive, maybe, but has the hard work done for you.

The RFM69HCW family of breakouts has a lot of library support and works quite well in pairs or a network.

If you know the time just before GPS signal loss and just after recovery then you can calculate the average speed of the interval. You would just have to maintain a little bit of GPS history.

To compliment GPS, you could do a simple pitot-static tube. It will calculate your airspeed. With 0 wind speed, your airspeed will equal your ground speed. In a tunnel, this could be the case much of the time.

Arctic_Eddie: I would first consider a GPS module with the added advantage of determining location.

What are you going to do with the RF modules?

thank you all for your replies I just want to design a RADAR GUN using arduino and radio receiver, transmitter.

So I just need help to get started to know about how to find speed using radio waves. (that's how police people use radar gun to find object speed);

Here's some info on how doppler radar works. Look at the equations as it pertains to the frequency of your modules.