It really depends on how class compliant that Focusrite product is. We've heard reports that Teensy does work with many products, but not all. Some apparently aren't quite as class compliant as they claim to be.
Teensy also implements a HID interface alongside the class compliant MIDI interface. Or it can implement a composite device of MIDI together with Serial and Audio. Some products expecting a class compliant MIDI USB device seem to be unable to ignore the extra non-MIDI interfaces. If you run into this, first try the various options in Tools > USB Type, since that is just an easy matter of clicking options in Arduino and uploading your program again.
If Focusrite iTrack Dock turns out to want a USB device having only a single MIDI interface and no other USB interfaces, you can achieve that with Teensy, but it does require editing usb_desc.h to get rid of the extra HID or Serial interface. Normally you would want those while developing your code, so Serial.print() can send to the Arduino Serial Monitor even while you're communicating MIDI messages with your PC.