Last time I was on the eastern Erie canal, the RR bridge at lock 19 was right at 21 feet, the lowest bridge on the eastern part. You can avoid the 17 foot in Chicago by taking the Calumet route instead of the Chicago river, but there is a 19 foot RR bridge on that route. That may be extended a little based on water tables, etc., no definitive answer on that. Do what you can to shorter below 19 feet and you will be OK.