Can I use a jigsaw to cut fence rails?
I own a Bosch JS470E and love it. I've owned many over my 20+ years in construction. It has a setting that will direct air from the motor to the area you're cutting and keeps saw dust clear from the line you're following.

"Will the jigsaw do the job"? Yes, with a good blade and little force but the job will go slow and the cut won't be as clean as a circular saw would do. If you won't use a circular saw that often I would suggest getting an inexpensive one. Maybe even go Harbor Freight. Then you can put some money into a good jigsaw for other uses.