how to convert railroad type???

[flatguy]

Could somebody tell me that how i can convert rail type to another in openttd???

[MeusH]

If you want to convert some track to i.e. monorail, you must open monorail building window, press the second-from-right button (converting).
Then you can drag-and-drop over tracks you want to change into monorail

[bobingabout]

doesn't work on trains. you have to sell them all, and re-create them manually.