Could airliners use a steam or electric powered catapult for take-off?
Let's see what the savings are:

A mid-sized airliner carries maybe 20% of its mass in fuel. This fuel has an energy density of 43 MJ per kg. Of that chemical energy at most 40% is converted into useable work. Heck, let's make this 25% so we are really conservative. Thus, the energy for the whole trip is $$E_{\text{trip}} = 0.2 \cdot 0.25 \cdot 43,000,000 \, \frac{\mathrm{J}}{\mathrm{kg}} \cdot \text{mass} = 2,150,000 \, \frac{\mathrm{J}}{\mathrm{kg}} \cdot \text{mass}$$

Now assume that this airliner saves the energy to accelerate from 0 to 150 knots by using a catapult. This energy is $$E_{\text{accel}} = \frac{v_{\text{takeoff}}^2 - v_0^2}{2} \cdot \text{mass} = 2,977.35 \,  \frac{\mathrm{m}^2}{\mathrm{s}^2} \cdot \text{mass}$$

Since I picked metric units, conversion is easy: $1 \, \mathrm{J} = 1 \, \mathrm{Ws} = 1 \, \frac{\mathrm{kg} \cdot \mathrm{m}^2}{\mathrm{s}^2}$. I use $\text{mass}$ for the take-off mass so you don't think it is the unit meter. Now let's put that into proportion:
$$\frac{E_{\text{accel}}}{E_{\text{trip}}} = 0.001385$$

Using the catapult saves 0.1385% of the energy needed to fly a typical airliner trip, assuming the same efficiencies during acceleration as during the flight. If we take into account that jet engines are most efficient during cruise, let's double the fuel need for acceleration and make it 0.277%. Granted, it is more for short-range flights, but still insignificant to what is needed to move the aircraft 10 km up into the sky and then for a couple of hundred miles through the air at Mach 0.8. In terms of fuel mass, these 0.277% are taken from 20% of the take-off mass. So the fuel needed to accelerate to v$_0$ is 0,000554 times take-off mass.

To make a catapult launch feasible, you need to add some strength to the nose gear and the forward fuselage. The typical landing gear fraction of the take-off mass is about 3%, and the nose gear is 10% - 15% of that, so $m_{\text{nosegear}} = 0.00375 \cdot \text{mass}$. Relative to the nose gear mass, the fuel saving from using a catapult launch is $\frac{0.000554}{0.00375} = 0.0148$ or 15% of the nose gear mass. Thus, the reinforcements need to add less than 15% to the mass of the nose gear.

If we assume an acceleration of ½ g = 4.903 m/s², the take-off run to accelerate to 150 kts is 607 m. I expect that even this moderate acceleration (which requires a pulling force of half of the lift at take-off) would translate into much higher mass increases than those 15% of the nose gear mass.