You can use them for colony ships before you get cryogenics, but cryogenics are so early on the tech tree it's usually not worth it.
Presenting the "Liberty" Class martian colonization vessel.
Liberty - Mod A class Luxury Liner 12,350 tons 341 Crew 482.1 BP TCS 247 TH 25 EM 0
101 km/s Armour 1-47 Shields 0-0 Sensors 5/1/0/0 Damage Control Rating 2 PPV 0
Maint Life 1.25 Years MSP 49 AFR 610% IFR 8.5% 1YR 33 5YR 488 Max Repair 6.25 MSP
Intended Deployment Time: 12 months Spare Berths 1
Passengers 500
12.5 EP Conventional Engine (2) Power 12.5 Fuel Use 87.34% Signature 12.5 Exp 12%
Fuel Capacity 150,000 Litres Range 2.5 billion km (286 days at full power)
Thermal Sensor TH1-5 (1) Sensitivity 5 Detect Sig Strength 1000: 5m km
This design is classed as a Military Vessel for maintenance purposes
What use is 500 passengers per trip you ask? With a round trip only taking about 45 days, and the colonization fleet increasing at a rate of 10 ships a year, mars is growing significantly. The 15 vessels in service can drop 60,000 people in a year, after 10 more ships that will be 100,000, etc.
Eventually I'll reach the point where the population is high enough to flag mars as a destination for trade, then I won't need to build or transport infrastructure there myself, I forget what population is required for that but I believe 100,000 is sufficient.
I'm curious to see if the shipping companies make luxury liners for colonist transport since trans newtonian technology is still 4 years away at this point.