As it turns out, if you type in 8458 in eBay's search box - someone is selling an 8458. Doris, that's a nice looking engine - a good find. It also turns out - as luck would have it - that someone else is also selling the instruction sheet for the GP-7, GP-9 and GP-20 diesels sold by Lionel (aka LTI or "Fundimensions") that year. Reading that sheet via the zoom feature on eBay, the reverse unit is electronic - so nevermind my earlier musings about the reversing unit. However, there is a sliding switch on the bottom of the chassis that is the 'lock-out' switch - designed to lockout the reversing sequence - so if it doesn't want to run in reverse at all, it might be that switch - or it could be dirty and making poor contacts internally. You could try moving the sliding switch several times and/or give it a squirt of contact cleaner. I think the switch wants to be in the 'forward' position (meaning toward the end of the loco with the 'short hood') to 'unlock' the reverse sequencing.