Computer should be no problem; most power supplies have a switch for voltage in the back, or are auto-sensing. Just switch to the US IEC cord and you're good to go. Of course, double-check the input ratings on the power supply (if not on the back of the case, it should be on a label just inside the case on the power supply if a tower computer)
Train set might be the trickier one, since it's going to be a variable DC transformer... Check the rating to see if it will accept 120; if not, replacing the transformer shouldn't set him back too much unless he has a really big setup.
If it's an incandescent light, it will WORK but will obviously be dimmer using 230V bulbs. Finding 120V bulbs that fit (assuming bayonet style) might be a challenge, so if he's really attached to the lamp, you could replace the socket... Or pick up a new lamp from Ikea or WalMart for $10.
