  We consider a soft quantum waveguide described by a two-dimensional
Schr\"odinger operators with an attractive potential in the form of a channel
of a fixed profile built along an infinite smooth curve which is not straight
but it is asymptotically straight in a suitable sense. Using Birman-Schwinger
principle we show that the discrete spectrum of such an operator is nonempty if
the potential well defining the channel profile is deep and narrow enough. Some
related problems are also mentioned.
