what sensor best suit in a railway to detect the train when it passes on it?

can you give me a suggestion on what kind of of sensor i will use on my project design, my design is all about on a automated railway, and i need a sensor that will detect the train when it passes over it...plzzzzzzzz help me....

please give some more info so we can help you better, like about how much the train weighs, how tall it is, if you have much space for it.

bluedemon said:

can you give me a suggestion on what kind of of sensor i will use on my project design, my design is all about on a automated railway, and i need a sensor that will detect the train when it passes over it...plzzzzzzzz help me....

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If the room has reasonable light, than just a photo cell should work good. As the train passes over it, the light to the photo cell will dim.

There are many sensors that might work, but I would choose something that doesn't give a false reading just because someone is walking on the track, or if a small vehicle passes on the track. You could use sound (noise) or a photosensor or weight/pressure or have a mechanical arm that gets touched by the train. I like the weight sensor idea. A strain gauge or similar kind of weight sensor would be buried under one of the rails and it would be calibrated to give an output when a very large weight is sensed, like several tons. Once buried, such a sensor would not be subject to breakage either.

Until you let us know if you're building the real deal or a model, it's hard to give you specific advice. In either situation, RadioRon's idea of a strain gauge is a good one. That said, don't fiddle with railway tracks unless you have explicit permission to do so.

Arguably the most conventional way would be to use an induction loop as used in detecting cars at street intersections. Read about them here:
https://auto.howstuffworks.com/red-light-camera1.htm
This is presuming you'll be putting your loop somewhere the roads for cars do not intersect the railway line. The induction loop will be able to accomplish the same task of counting only the trains and not anyone walking on the tracks.

Doesn't a train put out a signal, that's picked up by the crossing gates?

Real railway crossings work like this:
A shorting bar is wired across the tracks about 0.5 to 1Km from the crossing forming a large inductor. Wires are attached to each rail at the crossing and are attached to an oscillator. When the train passes the shorting bar, the axles short out the coil causing the oscillator to rise in frequency as the train approaches. It is this rise in frequency that sets off the crossing alarm circuitry which lowers the gates and rings the bells.

For model railways, a more reliable method than the photocell option is an infra-red emitter/receiver pair aimed from between the ties onto the bottom of the loco/carriages.