I recently flew IAH-EWR on CO 50.

Strangely enough, in EWR I saw a gate assigned to CO 50 to Hamburg (or Frankfurt or somewhere in Germany)

I though flight numbers could be assigned only once. This was clearly another flight; not a plane continuing on. Is there a difference in domestic / international?

There can only be one flight in the air at a time with a flight number. It is basically to keep ATC from going crazy.

CO has done this with most of their international flights. They make them "direct" from other cities around the country. Houston is the most common, but there are some from DEN, DCA and others. It used to help from a marketing perspective. Now it works against you because you only get the direct mileage, not including the cxn.

Many flights do this; they are routed through two or three cities via a hub. In many cases with international flights the flight changes equipment. CO is not the only airline who does this, they all do. By putting the two flights under a single flight number, it shows in reservation systems as a direct flight with a stop, which puts it up at the top of the list right under any nonstops but before connecting flights. It's a marketing ploy to improve the positioning of the flight in the reservation systems listings. Many frequent fliers dislike these flights because the airline awards mileage and elite points as if the flight was one flight, not a connection. So a passenger flying IAH-EWR-FRA gets one elite point in each direction, not two, and miles as if flying IAH-FRA nonstop (which does not exist on CO) as opposed to miles calculated as IAH-EWR and EWR-FRA.

So if my flight CO 50 were to be heavily delayed in IAH, say for 4 hours... they can't let the metal waiting in EWR that's flying to FRA leave before the delayed flight arrives... even though the EWR-FRA metal is technically OK?

No, they can let the onward portion go.

Generally the domestic portion is assigned a bogus flight number with ATC, and the major segment gets the flight. So IAH-EWR is probably designated some like CO 9899 and EWR-FRA is CO 50.

So anyone buying a ticket on CO 50 IAH-FRA (1-stop) may misconnect to the second portion of the flight.

Really the use of flight numbers in this manner is a marketing tool to make the flight look like it's a non-stop or direct one when it really isnt. It's basically a bunch of crap.

And to save CO from many FF miles, especially for CO6/7 and CO88/89. And CO99 BOS-EWR-HKG actually has fewer miles than EWR-HKG. You get to lose a lot of miles if book these trips as a "direct" flight.

The only positive thing I can say, based on my experience on a misconnect on a "direct" flight is that I got full NonePass� credit for the original flight as well as the flights that I was switched to.