This is a single field recording of a train horn (source below) being warped simultaneously in various ways by my modular synthesizer. One strand is going through a reverb, another through granular synthesis, another through a lo-fi looper, and another through a spectral filter, and then there’s the unadulterated line, and all of those are being warped or tweaked themselves. The shape of the overall sound, for example, is affecting the density of the granular synthesis. Various LFOs are adjusting the relative prominence of different elements, and other aspects such as the size of the reverb.
The source audio is from freesound.org. It is a distant train horn recorded by Andy Brannan in Wyoming.