No, not a bike for training. A bike train:
To build this, you’ll need as many steel-framed bikes as you’ve got friends who’re daft enough to want to ride the bike train. You could do it with aluminium bikes, but you’re going to be welding, which is easy-ish with steel, but the work of the Devil with aluminium.
Weld (see - I told you) a pair of bolts that are the same diameter as a standard bike axle to the top of the first bike’s drop-outs. Each bolt’s head gets welded to the drop-out, so that the bolt sticks out sideways by about an inch - just like an extra axle.
Then take the next bike, throw away the front wheel, and modify the forks (there’ll be some more welding involved here - don’t just bend ‘em) so that they’ll slip over the bolts that you welded onto the back of the first bike. Fix them in place with a pair of nuts on each.
Ta-Da! You’ve made an articulated tandem.
But wait - you can go further. Weld a pair of bolts onto the back of the second bike now. Then modify the forks of a third bike, and hook it up behind the second.
Repeat with bikes 4, 5, 6, 7 . . . until you run out of bikes or friends to pedal them.
So tell me - why couldn’t this work? OK, so the handling would probably be lethal on longer bike trains. Imagine coming up to a turning at speed - the rider of the front bike wants to lean into the turn, but can’t because of all the bikes he’s attached to, and they’re all still going in a straight line. The result - probably serious under-steer, or a jack-knife. If you manage to avoid that, then on the way out of the bend, the whole thing would probably start to fishtail like mad as each link tried to steer & over / under-compensate for things.
But it’d be great fun, wouldn’t it?