A Lunkenheimer Three Chime whistle. This particular one was recorded from Wilmington & Western #58. This variation is is the long-long-short-long pattern, though you can stop it or repeat it as you please (i.e. you can do a short blast or keep holding 'H' for the full thing). Download Variant A for the same whistle, but as a constant loop (i.e. you can hold it out as long as you want...even if it's for five minutes).