One important improvement for achieving greater effectiveness in train-activated warning systems at railroad-highway grade crossings (RHGC) would be to provide a constant warning time (CWT) to the motorist of the impending arrival of a train. This report describes an investigation that was carried out to identify, evaluate and demonstrate the feasibility of concepts upon which a general purpose CWT system could be developed. The scope of the study includes train detection, signal transmission, and associated logic, but did not include motorist warning devices. Primary emphasis was placed on the development of CWT concepts rather than equipment for such systems. Train detection techniques with the greatest potential for application to CWT systems are described and evaluated. These include seismic, magnetic, and acoustic transducers; doppler, guided and two dimensional radars, video sensors, strain gages, and proximity switches. The most promising of these are shown to be based on magnetic and acoustic concepts. Field tests carried out to demonstrate the feasibility of these techniques are described and the data is analyzed. It is shown that a great deal of further testing and development will be required before either of those techniques can be incorporated into a working CWT system.