A typical locomotive at 1 mph has more momentum than a Ford F-150 at 45 mph, but the latter is far more likely to kill you in collision so it doesn’t look like it is momentum that kills.

Why would that be? Isn't it the energy spent tearing your body apart that is relevant?

You are both wrong, It's the acceleration that kills you.

Doubling the mass of the car assuming everything else (crumple zones etc.) stays the same - doubles the acceleration experienced by the pedestrian.

    X_p0 = X of the pedestrian before collision
    X_c0 = X of the car before collision
    X_p1 = X of the pedestrian after the collision
    X_c1 = X of the car after the collision
    m_ = mass
    v_ = velocity
    a_ = acceleration
    t = time

    //conservation of momentum:
    m_p * v_p0 + m_c * v_c0 = m_p * v_p1 + m_c * v_c1
    m_p * (v_p0 - v_p1) = m_c * (v_c1 - v_c0)
    v_p1 - v_p0 = m_c * (v_c0 - v_c1) / m_p

    // a = delta v / t
    a = (v_p1 - v_p0)/t = m_c * (v_c0 - v_c1) / (m_p * t)

Acceleration, which is caused by a transfer of energy?