It might be possible to replace the horn sounds in your old GP40 with those of the new GP40. My program ADPCM can do that kind of sound file editing.
There are two limiting factors:
1. The EEPROM in PS2 engines is much smaller than the EEPROM in PS3 engines. The PS3 engine can hold more sounds (both in quantity and in length).
2. PS2 engines only supported a sound format called ADPCM (thus the name of my program). ADPCM is a 4 bit lossy IMA format that has a limited dynamic range and, being lossy, does not exactly reproduce the recorded sound. PS3 engines (as technology marches on...) supports a 24 bit signed PCM format. That format produces a much higher quality sound. You cannot put a 24 bit signed PCM audio clip in to a PS2 engine. It won't play.
More at: http://www.silogic.com/trains/ADPCM.html