Most, if not all modern motherboards made since several years usually have a sound chip on board which is normally very good in producing high quality sound for any game. One only needs to enable this sound in the BIOS setup of the PC. No external sound card is required with one of these motherboards. Unless you have an old PC which requires an external sound card to be used, then the older PC will most likely be not good enough for the late versions of Trainz anyway.
Sound card do not produce lag, this is your video card doing it, when it can not handle all the graphics (scenery and rolling stock) seen on your screen at a given moment. Any of the current mid range to top video cards should do the job. As you have not given us any of your PC's specs, it is only general advice one can give.
Lennard