What's the procedure for taking a bicycle on an overnight train? I was thinking of taking my bike to Laos. Is there any additional fees I should be aware of?

If it's an overnight train, it almost certainly has a baggage car. To take a bike, your train has to have a baggage car. Register your bike as baggage and pay a small fee at the station before boarding, then go to the baggage car and make sure you put your bike on yourself. I always take my own bungee cords and fasten the bike securely to the far wall, making sure the guy in charge understands not to jam anything up against it. So far the system has worked great. Just make sure you personally load the bike.

That's the easy part. Crossing the friendship bridge going north is hit and miss, many times they dont allow bikes. then you have to head to Tha li or bueng khan (sp?). heading home the friendship bridge is no problem. Tha li has laos visas on arrival, bueng khan does not, you need your visa first.

Well actually, I was thinking of taking a secon d train to thanaleng and then biking it from there. Presumably, the laotians are okay about this as I've never seen the train from nong kai to thanaleng full

I didn't know they were operating that yet. I saw the station in laos it's quite nice, but was empty. Please post how it goes, that would be a great way to go over.