  The filament IRDC G035.39--00.33 in the W48 molecular complex is one of the
darkest infrared clouds observed by \textit{Spitzer}. It has been observed by
the PACS (70 and 160\,$\micron$) and SPIRE (250, 350, and 500\,$\micron$)
cameras of the \textit{Herschel} Space Observatory as part of the W48 molecular
cloud complex in the framework of the HOBYS key programme. The observations
reveal a sample of 28 compact sources (deconvolved FWHM sizes $<$0.3 pc)
complete down to $\sim$$5 \msun$ in G035.39--00.33 and its surroundings. Among
them, 13 compact sources are massive dense cores with masses $>$$20 \msun$. The
cloud characteristics we derive from the analysis of their spectral energy
distributions are masses of $20-50 \msun$, sizes of 0.1--0.2 pc, and average
densities of $2-20 \times 10^{5} \cmc$, which make these massive dense cores
excellent candidates to form intermediate- to high-mass stars. Most of the
massive dense cores are located inside the G035.39--00.33 ridge and host
IR-quiet high-mass protostars. The large number of protostars found in this
filament suggests that we are witnessing a mini-burst of star formation with an
efficiency of $\sim$15% and a rate density of $\sim$$40
\msun\,$yr$^{-1}\,$kpc$^{-2}$ within $\sim$8 pc$^2$, a large area covering the
full ridge.
  Part of the extended SiO emission observed towards G035.39--00.33 is not
associated with obvious protostars and may originate from low-velocity shocks
within converging flows, as advocated by previous studies.
