A scramjet has to be going very fast, perhaps Mach 5+, in order to function. The scramjet tests that have been attempted to date have used rockets to reach that speed. However, that doesn't mean that airliners would have to do the same. You could use the SR-71 as an example of how it might work. First, you take off using turbojet or turbofan power. As the vehicle accelerates above Mach 2 or so, the engines converts over to a ramjet configuration. You keep accelerating using the ramjets until you're fast enough to light the scramjets. This whole process might take 20-30 minutes with no more acceleration than passengers currently experience on takeoff.

IMO, the real issues for developing a workable hypersonic airliner involve creating the heat resistant materials to withstand extremely high temperatures for upwards of two hours. There's also the issue of developing the engines that are efficient enough to allow the plane to carry a good payload, meet environmental requirements, and be profitable. NASA and Boeing spent a lot of time, money, and effort trying to develop the High Speed Commercial Transport (HSCT) and ended up putting the project on the shelves. It was Boeing's opinion that current technology didn't exist to develop the engines.

One problem with a hypersonic airliner is rather mundane. The plane would get so hot during the cruising phase that you might have to wait a long time before it'd be safe for the passengers to deplane. The SR-71 typically flew at subsonic speeds for some time before landing to let the plane cool. Doing that in a hypersonic airliner kind of defeats the purpose because you're wasting a lot of time during the cool down cycle. Parts of the SR-71 approached 1000 degrees Fahrenheit during cruise. A hypersonic airliner would get a lot hotter.