Well, CtP kind of did the trade route-trade unit thing, but didn't follow it through to its logical conclusion! Perhaps you could have a trade line, and a caravan moving along it, like a needle along a thread. Every time the 'unit' arrives at the destination, you recieve your 'payout'. The thing is that an enemy can discover the 'trade route', but would still need to find the unit along it. This wouldn't stop an enemy from placing a unit on a trade route in order to block the caravans progress. My sole problem with this is that it still doesn't solve the problem that we have as a result of turn lengths. Consider a 50 year turn, how many times could a caravan travel back and forth between the end of one turn and the beginning of the next. A possible solution would be to break the time between turns into 'semi-turns'. Each players turn equals a fraction of a whole turn. For instance, a 50 year turn might be broken into 5 year 'pseudo-turns' per player, which would help to determine where the trade unit currently is in its journey, for the purposes of attacking the caravan. Of course, a similar approach could be used for ANY unit. Increase overall movement rates significantly, but move the unit slightly each 'pseudo-turn'! Anyway, I'm kinda rambling here, but just trying to figure out how we could have a fusion of the two ideas. Ultimately, though, within a turn-based system like Civ, I think it is neccessary to have some concepts, like trade routes, abstracted in the way I have mentioned. It has the advantages of the unit system, without the difficulties mentioned above!
Yours,
Aussie_Lurker.